Rubuta ci gaba don wannan sakin layi - A ranar 7 ga Maris 2008, lokacin da Kumagai ke da shekaru 17, ta fara bugawa tawagar kasar Japan wasa da Canada. A watan Agusta, an zaɓi Kumagai don ƙungiyar 'yan wasan ƙasar Japan ta U-20 a gasar cin kofin duniya ta U-20 ta 2008. A shekara ta 2010, ta taka leda a kungiyar U-20 a matsayin kyaftin a lokacin gasar cin kofin duniya ta U-20 ta 2010.
Ka yi haƙuri. A shekara ta 2011, ta kasance wani ɓangare na tawagar da ta lashe gasar cin kofin duniya ta Japan, inda ta zira kwallaye a wasan karshe da Amurka. Ta kuma kasance a cikin tawagar a wasannin Olympics na bazara na 2012 da kuma gasar cin kofin duniya ta 2015. Japan ta zo ta biyu a duka gasar. A watan Janairun 2017, manajan Asako Takakura ya nada ta kyaftin din Japan. A shekarar 2018, Japan ta lashe Kofin Asiya na 2018. Ta buga wasanni sama da 100 don Japan. A ranar 10 ga Nuwamba, 2019, Kumagai ta zira kwallaye na farko a wasan sada zumunci ga Japan a nasarar 2<unk>0 da ta yi wa Afirka ta Kudu.