Conventionally, various heart rate meters have been suggested for measuring a heart rate of a living body. By way of example, the patent document 1 is known as disclosing a pulse detecting circuit which irradiates a light from a light emitting device onto a body, detects a reflected light or a transmitted light therefrom by a light receiving device, and converts a received signal into a pulse signal, thereby detecting the pulse.
In using the heart rate meter as described above, it is demanded that the pulse rate is stably displayed as against noise, and there are some suggestions for enhancing the stability of the pulserate display (e.g. patent document 2, patent document 3, and patent document 4).
The patent document 2 discloses a technique which focuses attention on the point that a pulse width caused by a noise is relatively narrow, and provides a pulse wave evaluation means between a pulse wave detecting circuit and a pulse wave operation means for evaluating the pulse width of pulse signals outputted from the pulse wave detecting circuit. With the pulse wave evaluation means, only a signal evaluated as a normal pulse wave signal is transferred to the pulse wave operation means, and thereby obtaining a stability in displaying the pulse.
The patent document 3 discloses a technique to remove a body motion by using an acceleration sensor and wavelet transformation. In this document, it is disclosed as the following: the acceleration sensor detects a body motion waveform assuming the body motion as an acceleration, and the body motion waveform is subjected to the wavelet transformation to generate body motion analytical data for each frequency domain; in addition, a pulse waveform detected from a detection target part of the living body is subjected to the wavelet transformation to generate pulse wave analytical data for each frequency domain; and the body motion analytical data is subtracted from the pulse wave analytical data, so as to detect a pulse.
The patent document 4 also discloses that in the photoplethysmography (PPG) for optically detecting heart rate information, a noise being a high frequency component, is removed from a PPG signal by using the wavelet transformation.    Patent document 1: Japanese Examined Patent Application Publication No. 61-29730    Patent document 2: Japanese Examined Patent Application Publication No. 04-79250    Patent document 3: Japanese Unexamined Patent Application Publication No. 11-9564    Patent document 4: Japanese Examined Patent Application Publication No. 2003-310562    Non-patent document 1: “Exercise Physiology for Health and Physical Fitness”, pp. 75 to 78, Author: Toshihiro Ishikawa, Professor emeritus, Juntendo University, published by Kyorin-shoin, April 2000