As this type of controller, there is known a controller applied to a contactless power supply system including a primary side coil provided in a power supply station and constituting a primary side resonance circuit and a secondary side coil provided in a vehicle and constituting a secondary side resonance circuit, as seen in the following patent literature 1. This controller is provided to suppress the deviation of a charging voltage of a battery connected to the output side of the secondary side resonance circuit from a target voltage. The deviation is due to a change in the gap between the primary side coil and the secondary side coil.
More specifically, prior to the start of charging of the battery, the battery is electrically disconnected from the output side of the secondary side resonance circuit, and a resistor is connected in parallel to the output side of the secondary side resonance circuit. Then, an output voltage of a high frequency power source for applying an AC voltage to the primary side resonance circuit is set to a measuring voltage, and an output current of the high frequency power source in this case is measured. Since the resistance of the resistor is known, a proper output voltage of the high frequency power source for setting the charging voltage of the battery to the target voltage can be calculated based on the measuring voltage and the output current. Because of this, the deviation of the charging voltage of the battery and the target voltage due to a change in the gap between the primary side coil and the secondary side coil is suppressed, and a decrease in the power transmission efficiency between the coils is suppressed.