Power transmitting device and wireless power transmission system

A power transmitting device for use in a wireless power transmission system that includes the power transmitting device and a power receiving device, the power transmitting device comprising:

• an inverter circuit;
• a power transmitting antenna connected to the inverter circuit, and being electromagnetically coupled to a power receiving antenna in the power receiving device to wirelessly transmit electric power thereto;
• a detector to detect an output voltage and an output current of the inverter circuit;
• and
• a control circuit to control the inverter circuit, the control circuit consecutively driving the inverter circuit at a plurality of frequencies, determining from among the plurality of frequencies a frequency at which a phase difference that is indicative of a lag of a phase of the output current relative to a phase of the output voltage becomes largest, and performing power transmission by driving the inverter circuit at an operating frequency that is based on the determined frequency.