A wireless power feeder for feeding power by wireless from a feeding coil to a receiving coil, comprising: a power transmission control circuit that feeds AC power to the feeding coil; a feeding coil circuit that includes the feeding coil and a capacitor and resonates at the resonance frequency of the receiving coil; and a phase detection circuit that detects a phase difference between the voltage phase and current phase of the AC power, wherein the phase detection circuit includes a current phase detection circuit and a voltage phase detection circuit; the voltage phase detection circuit generates a signal indicating a phase value of a voltage phase of the AC power and includes a voltage waveform shaping circuit, a voltage reference adjustment circuit and a comparator, the voltage waveform shaping circuit shapes a voltage component of the AC power into a voltage phase signal of a saw-tooth waveform, the voltage reference adjustment circuit outputs a first reference value, and the comparator compares the voltage phase signal and the first reference value and outputs the signal indicating the phase value of the voltage phase of the AC power; the current phase detection circuit generates a signal indicating a phase value of a current phase of the AC power and includes a current waveform shaping circuit, a current reference adjustment circuit and a comparator, the current waveform shaping circuit shapes a current component of the AC power into a current phase signal of a saw-tooth waveform, the current reference adjustment circuit outputs a second reference value, and the comparator compares the current phase signal and the second reference value and outputs the signal indicating the phase value of the current phase of the AC power; the power transmission control circuit adjusts the drive frequency of the AC power such that the phase difference is reduced so as to make the drive frequency track the resonance frequency, in the phase detection circuit, a detected phase value of both or one of the voltage and current phases can be changed intentionally so as to intentionally create a resonance state or a non-resonance state, the phase detection circuit compares 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 by changing both or one of the first and second reference values, both or one of the first and second phase values are changed, the phase detection circuit converts both or one of voltage and current components of the AC power into a signal having a saw-tooth waveform for detection of the phase difference. a power transmission control circuit that feeds AC power to the feeding coil; a feeding coil circuit that includes the feeding coil and a capacitor and resonates at the resonance frequency of the receiving coil; and a phase detection circuit that detects a phase difference between the voltage phase and current phase of the AC power, wherein the phase detection circuit includes a current phase detection circuit and a voltage phase detection circuit; the voltage phase detection circuit generates a signal indicating a phase value of a voltage phase of the AC power and includes a voltage waveform shaping circuit, a voltage reference adjustment circuit and a comparator, the voltage waveform shaping circuit shapes a voltage component of the AC power into a voltage phase signal of a saw-tooth waveform, the voltage reference adjustment circuit outputs a first reference value, and the comparator compares the voltage phase signal and the first reference value and outputs the signal indicating the phase value of the voltage phase of the AC power; the current phase detection circuit generates a signal indicating a phase value of a current phase of the AC power and includes a current waveform shaping circuit, a current reference adjustment circuit and a comparator, the current waveform shaping circuit shapes a current component of the AC power into a current phase signal of a saw-tooth waveform, the current reference adjustment circuit outputs a second reference value, and the comparator compares the current phase signal and the second reference value and outputs the signal indicating the phase value of the current phase of the AC power; the power transmission control circuit adjusts the drive frequency of the AC power such that the phase difference is reduced so as to make the drive frequency track the resonance frequency, in the phase detection circuit, a detected phase value of both or one of the voltage and current phases can be changed intentionally so as to intentionally create a resonance state or a non-resonance state, the phase detection circuit compares 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 by changing both or one of the first and second reference values, both or one of the first and second phase values are changed, the phase detection circuit converts both or one of voltage and current components of the AC power into a signal having a saw-tooth waveform for detection of the phase difference.