A wireless power feeder that feeds power from a power feeding coil at a power feeding side to a power receiving coil at a power receiving side by wireless using a magnetic field resonance phenomenon between the power feeding coil and power receiving coil, said feeder comprising: the power feeding coil; a power transmission control circuit arranged to feed AC power to the power feeding coil at a drive frequency so as to make the power feeding coil feed the AC power to the power receiving coil; a phase detection circuit arranged to detect a phase difference between voltage and current phases of the AC power; a signal receiving coil; and a signal adjustment circuit arranged to DC-convert an output signal in accordance with a duty ratio, wherein the power transmission control circuit is arranged to adjust the drive frequency so as to reduce the phase difference; the phase detection circuit is arranged to perform ex-post adjustment of the detected value of both or one of the voltage and current phases in accordance with a signal level of the DC-converted output signal; and the coil plane of the signal receiving coil and that of the power feeding coil are substantially the same, characterized in that the signal receiving coil is provided inside the power feeding coil; a reference frequency (fs) indicating a frequency of the output signal is higher than a resonance frequency of the power feeding coil and the power receiving coil; the signal receiving coil is arranged to receive from the power receiving side, the output signal indicating an output level by the duty ratio; and the phase detection circuit is arranged to compare a first phase value indicating a timing at which the voltage level of the AC power becomes a first reference value and a second phase value indicating a timing at which the current level of the AC power becomes a second reference value to detect the phase difference and is arranged to change both or one of the first and second reference values in accordance with the signal level to perform ex-post adjustment of both or one of the first and second phase values. the power feeding coil; a power transmission control circuit arranged to feed AC power to the power feeding coil at a drive frequency so as to make the power feeding coil feed the AC power to the power receiving coil; a phase detection circuit arranged to detect a phase difference between voltage and current phases of the AC power; a signal receiving coil; and a signal adjustment circuit arranged to DC-convert an output signal in accordance with a duty ratio, wherein the power transmission control circuit is arranged to adjust the drive frequency so as to reduce the phase difference; the phase detection circuit is arranged to perform ex-post adjustment of the detected value of both or one of the voltage and current phases in accordance with a signal level of the DC-converted output signal; and the coil plane of the signal receiving coil and that of the power feeding coil are substantially the same, characterized in that the signal receiving coil is provided inside the power feeding coil; a reference frequency (fs) indicating a frequency of the output signal is higher than a resonance frequency of the power feeding coil and the power receiving coil; the signal receiving coil is arranged to receive from the power receiving side, the output signal indicating an output level by the duty ratio; and the phase detection circuit is arranged to compare a first phase value indicating a timing at which the voltage level of the AC power becomes a first reference value and a second phase value indicating a timing at which the current level of the AC power becomes a second reference value to detect the phase difference and is arranged to change both or one of the first and second reference values in accordance with the signal level to perform ex-post adjustment of both or one of the first and second phase values.