A chewing detecting device includes: earphone-type external auditory meatus sensors which have a pair of a light emitting element and a light receiving element and in which the light receiving element receives reflective light of light emitted by the light emitting element into an external auditory meatus to output a voltage signal corresponding to a light receiving amount； association processing means associating an output signal of the external auditory meatus sensors with a motion of a jaw, and outputting a chewing signal showing that the jaw performs chewing； and chewing section sensing means which determines whether or not an output of the external auditory meatus sensors is based on the motion of the jaw (within a chewing section), and which invalidates the output of the association processing means when the output of the external auditory meatus sensors is not based on the motion of the jaw (without the chewing section).
