A problem to be solved by the present invention is that there is much noise in a signal which is obtained with an optical sensor, and it is not possible to properly detect chewing. Provided is a chewing detecting device, comprising: earphone-type external auditory canal sensors 11, 12, which comprise combinations of light-emitting elements 34 and photoreceptor elements 35, in which the photoreceptor elements 35 receive reflected light of light which the light-emitting elements 34 have emitted within the external auditory canals, and which output piezoelectric signals corresponding to the received light quantities associating means 51 for associating output signals of the external auditory canal sensors 11, 12 with jaw actions, and outputting chewing signals to the effect that the jaw has made a chewing action and chewing period sensing means 52 for assessing whether the output of the external auditory canal sensors 11, 12 is based on the jaw actions (within a chewing period), and when not based on the jaw actions (outside the chewing period), invalidating the output of the associating means 51.
