WIRELESS POWER TRANSFER SYSTEM

In a wireless power transfer system, a resonance circuit is formed only on the secondary coil side, phase information of a resonance current flowing in the resonance circuit is detected, and, base on this phase information, a driving frequency is determined so that the current phase of a driving current flowing in a primary coil slightly delays from the voltage phase, thereby driving the primary coil. A Q value determined based on a leakage inductance of the secondary coil, a capacitance of a resonance capacitor, and an equivalent load resistance is set to a value greater than or equal to a value determined by Q=2/k2 (k is a coupling coefficient).