An activity state information detecting device includes: a pulse wave measuring section that measures pulse wave information of a user; a body motion measuring section that measures body motion information of the user; and a processing section that performs a calculation process of activity state information of the user, in which the processing section performs a mode switching process between a first mode for performing the calculation process of the activity state information based on the body motion information and a second mode for performing the calculation process of the activity state information based on the pulse wave information.