hereinafter , embodiments of the present invention will be described with reference to the drawings . it should be noted , however , that the present invention is not limited to the following examples . fig1 , 2 a and 2 b illustrate a first embodiment of a golf club shaft according to the present invention . in fig2 a and 2b , a grip 11 is not illustrated . a shaft 10 of the first embodiment includes an outer shaft 12 and an inner shaft 14 disposed in a hollow portion of the outer shaft 12 . a void portion 16 is formed between the outer shaft 12 and the inner shaft 14 . in the first embodiment , each of the outer shaft 12 and the inner shaft 14 has a hollow tapered conical shape such that the diameter is gradually reduced toward the tip end . the outer shaft 12 and the inner shaft 14 have almost equal lengths . the outer shaft 12 preferably has a length with a lower limit of approximately 787 mm and an upper limit of approximately 1219 mm . the outer shaft 12 preferably includes a distal end having an outer diameter with a lower limit of approximately 8 . 4 mm and an upper limit of approximately 10 mm ; a proximal end having an outer diameter with a lower limit of approximately 14 mm and an upper limit of approximately 17 mm ; and a wall portion having a thickness with a lower limit of approximately 0 . 6 mm and an upper limit of approximately 2 . 5 mm . the inner shaft 14 preferably has a length with a lower limit of approximately 787 mm and an upper limit of approximately 1219 mm . the inner shaft 14 preferably includes a distal end having an outer diameter with a lower limit of approximately 1 mm and an upper limit of approximately 4 mm ; a proximal end having an outer diameter with a lower limit of approximately 4 mm and an upper limit of approximately 15 . 8 mm ; and a wall portion having a thickness with a lower limit of approximately 0 . 6 mm and an upper limit of approximately 4 mm . the void portion 16 has a width a , at the distal end , with a lower limit of preferably approximately 1 mm , particularly preferably approximately 2 mm , and an upper limit of preferably approximately 6 mm , particularly preferably approximately 4 mm . moreover , the width a of the void portion 16 is gradually increased from the proximal end of the golf club shaft toward the distal end thereof . thus , the width a is larger at the distal end of the golf club shaft than the proximal end thereof . this produces such an operation and effect that flexure at the distal end of the golf club shaft can be utilized to a great extent ( in a long range ). a proximal end portion of the outer shaft 12 is connected to a proximal end portion of the inner shaft 14 through a connecting member 18 . this prevents the inner shaft 14 from coming off from the outer shaft 12 . examples of the material of the connecting member 18 include rubbers , resins , elastomers , corks , woods , and the like . moreover , a vibration absorbing member 20 is disposed in the void portion 16 at a position between a distal end portion of the outer shaft 12 and a distal end portion of the inner shaft 14 . this prevents the outer shaft 12 and the inner shaft 14 from making a vibration sound ( reverberating noise ) during a swing . examples of the material of the vibration absorbing member 20 include : elastic materials such as rubbers and elastomers ; foamed products thereof such as soft sponges made of polyurethane foam ; and similar materials . as the material of the outer shaft 12 and the inner shaft 14 , a fiber - reinforced resin or a metal can be used . in this case , examples of reinforced fibers in the fiber - reinforced resin include carbon fibers , glass fibers , metal fibers , aramid fibers , silicon carbide fibers , alumina fibers , boron fibers , and the like . examples of a thermosetting resin in the fiber - reinforced resin include epoxy resins , phenol resins , unsaturated polyester resins , and the like . as a subsidiary material of the fiber - reinforced resin , a curing agent , a curing accelerator , a filler , a release agent , a pigment , and the like can be used . furthermore , examples of the metal include iron alloys , titanium alloys , and the like . note that the inner shaft 14 can be formed only of the resin . as the outer shaft 12 and the inner shaft 14 , it is possible to use shafts having multiple fiber - reinforced resin layers in which reinforced fibers are arranged in one direction . the fiber - reinforced resin layer can be formed by , for example , a sheet winding method in which a prepreg sheet is wound around a mandrel and is subjected to heating for curing . as the prepreg sheet , it is possible to use reinforced fibers such as a cloth , mat , or a roving aligned in parallel which are impregnated with the thermosetting resin . in the shaft 10 of the first embodiment , the outer shaft 12 plays a major role , while the inner shaft 14 plays a minor role . thus , for the outer shaft 12 , the fiber - reinforced resin layers are preferably mainly biased layers in which fibers are arranged obliquely to the axis of the shaft . meanwhile , for the inner shaft 14 , the fiber - reinforced resin layers are preferably mainly longitudinal layers in which fibers are arranged in the same direction as the axis of the shaft . note that , when a shaft is formed using a mandrel described above , a releasing agent is applied to the surface of the mandrel . accordingly , the release agent is attached to an inner surface of the shaft . it is preferable to remove the release agent by polishing the inner surface of the shaft . fig3 a and 3b illustrate a second embodiment of a golf club shaft according to the present invention . in fig3 a and 3b , a grip 11 is not illustrated . a shaft 30 of the second embodiment includes an outer shaft 12 and an inner shaft 14 disposed in a hollow portion of the outer shaft 12 . a void portion 16 is formed between the outer shaft 12 and the inner shaft 14 . in the second embodiment , the outer shaft 12 has such a hollow tapered conical shape that the diameter is gradually reduced toward the tip end . the inner shaft 14 has such a hollow cylindrical shape that the diameter at the proximal end is the same as the diameter at the distal end . the outer shaft 12 and the inner shaft 14 have almost equal lengths . the outer shaft 12 and the inner shaft 14 may each have a length , outer diameter , and thickness of the wall portions which are the same as those in the first embodiment . the void portion 16 has a width a , at the distal end , with a lower limit of preferably approximately 1 mm , particularly preferably approximately 2 mm , and an upper limit of preferably approximately 6 mm , particularly preferably approximately 4 mm . moreover , the width a of the void portion 16 is gradually reduced from the proximal end of the golf club shaft toward the distal end thereof . thus , the width a is smaller at the distal end of the golf club shaft than the proximal end thereof . this produces such an operation or effect that the inner shaft can be reduced in weight . in the second embodiment , the functions and materials of a connecting member 18 and a vibration absorbing member 20 , the materials and forming methods of the outer shaft 12 and the inner shaft 14 , and the like are the same as those in the first embodiment . accordingly , the descriptions will be omitted . fig4 a and 4b illustrate a third embodiment of a golf club shaft according to the present invention . in fig4 a and 4b , a grip 11 is not illustrated . a shaft 40 of the third embodiment includes an outer shaft 12 and an inner shaft 14 disposed in a hollow portion of the outer shaft 12 . a void portion 16 is formed between the outer shaft 12 and the inner shaft 14 . in the third embodiment , the outer shaft 12 has such a hollow tapered conical shape that the diameter is gradually reduced toward the tip end . the inner shaft 14 has such a solid cylindrical shape that the diameter at the proximal end is the same as the diameter at the distal end . the outer shaft 12 and the inner shaft 14 have almost equal lengths . the outer shaft 12 may have the same length , outer diameter , and thickness of the wall portions as that in the first embodiment . the inner shaft 14 may have the same length and outer diameter as that in the first embodiment . the void portion 16 has a width a , at the distal end , with a lower limit of preferably approximately 1 mm , particularly preferably approximately 2 mm , and an upper limit of preferably approximately 6 mm , particularly preferably approximately 4 mm . moreover , the width a of the void portion 16 is gradually reduced from the proximal end of the golf club shaft toward the distal end thereof . thus , the width a is smaller at the distal end of the golf club shaft than the proximal end thereof . this produces the same operation or effect as in the second embodiment . in this embodiment , the functions and materials of a connecting member 18 and a vibration absorbing member 20 , the material and forming method of the outer shaft 12 , the material of the inner shaft 14 , and the like are the same as those in the first embodiment . accordingly , the descriptions will be omitted . the inner shaft 14 can be formed by a known forming method using a fiber - reinforced resin or a metal . fig5 a , 5 b , and 5 c illustrate a fourth embodiment of a golf club shaft according to the present invention . in fig5 a , 5 b , and 5 c , a grip 11 is not illustrated . a shaft 50 of the fourth embodiment includes an outer shaft 12 and an inner shaft 14 disposed in a hollow portion of the outer shaft 12 . a void portion 16 is formed between the outer shaft 12 and the inner shaft 14 . in the fourth embodiment , the outer shaft 12 has such a hollow tapered conical shape that the diameter is gradually reduced toward the tip end . the inner shaft 14 has on its proximal end side such a hollow tapered conical shape portion 12 a that the diameter is gradually reduced toward the tip end , and has on its distal end side such a solid cylindrical shape portion 12 b that the diameter at the proximal end is the same as the diameter at the distal end . the outer shaft 12 and the inner shaft 14 have almost equal lengths . the outer shaft 12 may have the same length , outer diameter , and thickness of the wall portions as that in the first embodiment . the inner shaft 14 may have the same length and outer diameter as that in the first embodiment . the void portion 16 preferably has a width a , at the distal end , with a lower limit of preferably approximately 1 mm , particularly preferably approximately 2 mm , and an upper limit of preferably approximately 6 mm , particularly preferably approximately 4 mm . moreover , the width a of the void portion 16 is gradually increased on the proximal end side from the proximal end of the golf club shaft toward the distal end thereof , and is gradually reduced on the distal end side from the proximal end of the golf club shaft toward the distal end thereof . this produces an operation or effect such that the inner shaft can be reduced in weight . in the fourth embodiment , the functions and materials of a connecting member 18 and a vibration absorbing member 20 , the material and forming method of the outer shaft 12 , the material of the inner shaft 14 , and the like are the same as those in the first embodiment . accordingly , the descriptions will be omitted . the inner shaft 14 can be formed by a known forming method using a fiber - reinforced resin or a metal . it should be noted that the golf club shaft of the present invention is not limited to the above embodiments . for example , in the embodiments , the length of the outer shaft is equal to the length of the inner shaft ; however , both of the shafts may have different lengths . in addition , in the embodiments , the inner shaft has a circular section , but may have other shapes such as a polygon . next , operations and effects of the shaft of the present invention will be described with reference to fig6 and 7 . fig6 is an explanatory view showing how the shaft 40 of the third embodiment flexes when a golfer slow in head speed swings a golf club with the shaft . as shown in fig6 , the outer shaft 12 flexes , but the inner shaft 14 does not flex at all or flexes little depending on the head speed . as a result , the shaft 40 is flexible and suitable for a golfer slow in head speed . the flexibility of the shaft 40 is utilized to increase the launch angle of a ball ; thereby , the driving distance can be increased . fig7 is an explanatory view showing how the shaft 40 of the third embodiment flexes when a golfer fast in head speed swings the golf club with the shaft . as shown in fig7 , both of the outer shaft 12 and the inner shaft 14 flex in accordance with the head speed , although the degree of the flexure varies depending on the head speed . as a result , the shaft 40 is stiff and suitable for a golfer fast in head speed . the stiffness of the shaft is utilized to increase the initial speed of a ball ; thereby , the driving distance can be increased . note that , in this case also , the outer shaft 12 plays a major role in the shaft 40 , while the inner shaft 14 plays a minor role . note that , although the shaft of the third embodiment is used to describe the operations and the effects of the shaft of the present invention , it would be easily understood by those skilled in the art that even the shafts in the other embodiments can exhibit the same operations and effects . the descriptions have been given of the embodiments of the golf club shafts and the golf club according to the present invention . however , the present invention is not limited to these embodiments . alterations and modifications apparent to those skilled in the art are included within the entire technical scope of the present invention .
a golf club shaft includes an outer shaft and an inner shaft disposed in a hollow portion of the outer shaft . a void portion is formed between the outer shaft and the inner shaft . the golf club shaft has a double structure including the outer shaft and the inner shaft . this allows both of the shafts to integrally operate and also allows one of the shafts to primarily operate .