There is a problem that a signal obtained by an optical sensor contains many noises and a chewing action cannot be precisely detected. A chewing detecting device includes: earphone-type external auditory meatus sensors 11, 12 which have a pair of a light emitting element 34 and a light receiving element 35 and in which the light receiving element 35 receives reflective light of light emitted by the light emitting element 34 into an external auditory meatus to output a voltage signal corresponding to a light receiving amount association processing means 51 associating an output signal of the external auditory meatus sensors 11, 12 with a motion of a jaw, and outputing a chewing signal showing that the jaw performs chewing and chewing section sensing means 52 which determines whether or not an output of the external auditory meatus sensors 11, 12 is based on the motion of the jaw (within a chewing section), and which invalidates the output of the association processing means 51 when the output of the external auditory meatus sensors 11, 12 is not based on the motion of the jaw (without the chewing section).
