A device for detecting a chewing action performed by a person, the chewing detecting device including: an earphone-type external auditory meatus sensor (11, 12) which has a pair of a light emitting element and a light receiving element and in which the light receiving element is configured to receive 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 for associating an output signal of the external auditory meatus sensor with a motion of a jaw, and for outputting a chewing signal showing that the jaw performs chewing; characterized in that the chewing detecting device further comprises a converter configured to convert the output of the external auditory meatus sensor into a digital signal, wherein the converter is configured to perform a signal conversion so as to output a predetermined number of sample values in a predetermined time; and chewing section sensing means which is configured to determine whether or not an output of the external auditory meatus sensor (11, 12) is based on the motion of the jaw, and which is configured to invalidate the output of the association processing means when the output of the external auditory meatus sensor (11, 12) is not based on the motion of the jaw, wherein the chewing section sensing means has a determination sensing algorithm which determines the chewing section when a constant rate or more of sample values among a constant number of sample values input from the converter within a predetermined sensing section is a significant value, and determines not the chewing section when not more than the constant rate of sample values is a significant value. an earphone-type external auditory meatus sensor (11, 12) which has a pair of a light emitting element and a light receiving element and in which the light receiving element is configured to receive 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 for associating an output signal of the external auditory meatus sensor with a motion of a jaw, and for outputting a chewing signal showing that the jaw performs chewing; characterized in that the chewing detecting device further comprises a converter configured to convert the output of the external auditory meatus sensor into a digital signal, wherein the converter is configured to perform a signal conversion so as to output a predetermined number of sample values in a predetermined time; and chewing section sensing means which is configured to determine whether or not an output of the external auditory meatus sensor (11, 12) is based on the motion of the jaw, and which is configured to invalidate the output of the association processing means when the output of the external auditory meatus sensor (11, 12) is not based on the motion of the jaw, wherein the chewing section sensing means has a determination sensing algorithm which determines the chewing section when a constant rate or more of sample values among a constant number of sample values input from the converter within a predetermined sensing section is a significant value, and determines not the chewing section when not more than the constant rate of sample values is a significant value.