A wireless power feeder 116 feeds power by wireless from a feeding coil L2 to a receiving coil L3 using a magnetic field resonance phenomenon. A power transmission control circuit 200 supplies AC power at a drive frequency fo to the feeding coil L2, thereby making the feeding coil L2 feed the AC power to the receiving coil L3. A phase detection circuit 150 detects the phase difference between the voltage phase and current phase of the AC power. Concretely, the phase detection circuit compares a first detection time period during which a signal T2 assumes a high level and a second detection time period during which a signal S2 assumes a high level and detects the length of the time period in which the first and second detection periods overlap each other to detect the phase difference.