A power feeding system has a power receiving device having: a resonant circuit having a receiving coil, a resonant capacitor configured to resonate with the receiving coil, and a first switching element configured to change an electrical connection state of the resonant capacitor; and a resonance control unit configured to control the first switching element, and has a power feeding device having: a second switching element connected in series to a feeding coil; a drive signal generation unit configured to generate a drive signal for driving the feeding coil; a variation detection unit configured to detect the change in electrical connection state of the resonant capacitor; and a power feeding control unit configured to control electric power supplied to the feeding coil based on a period of the resonant state and a period of the non-resonant state that are estimated from the periodic waveform variation.