A wireless power feeder for feeding power by wireless from a feeding coil (L2) to a receiving coil (L3) at a resonance frequency (fr) of the feeding coil (L2) and receiving coil (L3), comprising: a resonance circuit that includes a first coil (L2) and a capacitor (C1) which are connected in series; a first switch (Q1) adapted to control supply of power fed via a first current path to the resonance circuit, wherein the first current path is a path through the first switch (Q1); a second switch (Q2) adapted to control supply of power fed via a second current path to the resonance circuit, wherein the second current path is a path through the second switch (Q2); a power transmission control circuit adapted to make the first and second switches (Q1, Q2) alternately conductive to cause the resonance circuit to resonate to transmit AC power from the first coil (L2) serving as the feeding coil (L2) to the receiving coil (L3); a second coil (LSS) adapted to generate inductive current (ISS) using a magnetic field generated by the AC power; and a phase detection circuit adapted to detect a phase difference between voltage phase and current phase of the AC power, wherein the phase detection circuit is adapted to measure the phase of the inductive current (ISS) flowing in the second coil (LSS) to achieve measurement of the current phase of the AC current, characterized in that the power transmission control circuit is an oscillator adapted to generate AC voltage at a drive frequency (f0); the oscillator is adapted to feed the AC voltage alternately to the first switch (Q1) and the second switch (Q2) ; the oscillator output is connected to a phase detection circuit input; and the phase detection circuit is adapted to measure the phase of the AC voltage generated by the oscillator to achieve measurement of the voltage phase of the AC power. a resonance circuit that includes a first coil (L2) and a capacitor (C1) which are connected in series; a first switch (Q1) adapted to control supply of power fed via a first current path to the resonance circuit, wherein the first current path is a path through the first switch (Q1); a second switch (Q2) adapted to control supply of power fed via a second current path to the resonance circuit, wherein the second current path is a path through the second switch (Q2); a power transmission control circuit adapted to make the first and second switches (Q1, Q2) alternately conductive to cause the resonance circuit to resonate to transmit AC power from the first coil (L2) serving as the feeding coil (L2) to the receiving coil (L3); a second coil (LSS) adapted to generate inductive current (ISS) using a magnetic field generated by the AC power; and a phase detection circuit adapted to detect a phase difference between voltage phase and current phase of the AC power, wherein the phase detection circuit is adapted to measure the phase of the inductive current (ISS) flowing in the second coil (LSS) to achieve measurement of the current phase of the AC current, characterized in that the power transmission control circuit is an oscillator adapted to generate AC voltage at a drive frequency (f0); the oscillator is adapted to feed the AC voltage alternately to the first switch (Q1) and the second switch (Q2) ; the oscillator output is connected to a phase detection circuit input; and the phase detection circuit is adapted to measure the phase of the AC voltage generated by the oscillator to achieve measurement of the voltage phase of the AC power. <EOS>