Provided is a power-receiving device according to at least one embodiment includes: a transmission/reception unit (2) including a resonant circuit having an antenna (2a) configured to receive a power transmitted from a contactless charging device (50) and to transmit and receive data to and from the contactless charging device (50); a charge control unit (5) configured to perform power source control; and a control unit (3) configured to generate control state data indicating a charging state of the secondary cell (6) and/or a power source state of an apparatus main unit (30) operated with the secondary cell (6) and to transmit the control state data to the contactless charging device (50). The control unit (3) has a power-receiving mode in which normal power supply is performed and a regulation mode in which the received power is regulated by regulating a resonant frequency of the resonant circuit.