In a resonant frequency control method in a magnetic field resonant coupling type power transmission system transmitting an electric power from a power transmitting coil to a power receiving coil using magnetic field resonance, a high-speed, accurate and real-time adjustment of the resonant frequency of a coil is realized. The phase of a voltage supplied to a power transmitting coil and the phase of a current that flows in the power transmitting coil or a power receiving coil is detected and the resonant frequency of the power transmitting coil or the power receiving coil is varied such that the phase difference between them becomes a target value.