A wireless power feeder adapted to feed power by wireless from a feeding coil at a power feeder side to a receiving coil at a power receiving side, comprising: a power transmission control circuit configured to feed AC power to the feeding coil at a drive frequency; a feeding coil circuit that includes the feeding coil and a capacitor and that is configured to resonate at a resonance frequency of the receiving coil; a phase detection circuit configured to detect a phase difference between a voltage phase and a current phase of the AC power; and a signal receiving circuit adapted to receive an output signal indicating the magnitude of an output voltage applied to a part of a loading circuit of a wireless power receiver from the power receiving side of the AC power, wherein the power transmission control circuit is configured to adjust the drive frequency so as to reduce the phase difference to make the drive frequency track the resonance frequency of the receiving coil, and the phase detection circuit is configured to perform ex-post adjustment of the detected value of both or one of the voltage and current phases according to the received output signal, wherein the phase detection circuit is configured to compare a first phase value indicating a timing at which a voltage level of the AC power becomes a first reference value and a second phase value indicating a timing at which a current level of the AC power becomes a second reference value to detect the phase difference and to change both or one of the first and second reference values based on the received output signal to perform ex-post adjustment of both or one of the first and second phase values. a power transmission control circuit configured to feed AC power to the feeding coil at a drive frequency; a feeding coil circuit that includes the feeding coil and a capacitor and that is configured to resonate at a resonance frequency of the receiving coil; a phase detection circuit configured to detect a phase difference between a voltage phase and a current phase of the AC power; and a signal receiving circuit adapted to receive an output signal indicating the magnitude of an output voltage applied to a part of a loading circuit of a wireless power receiver from the power receiving side of the AC power, wherein the power transmission control circuit is configured to adjust the drive frequency so as to reduce the phase difference to make the drive frequency track the resonance frequency of the receiving coil, and the phase detection circuit is configured to perform ex-post adjustment of the detected value of both or one of the voltage and current phases according to the received output signal, wherein the phase detection circuit is configured to compare a first phase value indicating a timing at which a voltage level of the AC power becomes a first reference value and a second phase value indicating a timing at which a current level of the AC power becomes a second reference value to detect the phase difference and to change both or one of the first and second reference values based on the received output signal to perform ex-post adjustment of both or one of the first and second phase values.