A wireless power transmission system includes a power transmitting device, power receiving device, and load. The power transmitting device includes an inverter circuit, power transmitting antenna, power transmission control circuit, and transmitting-side receiver. The power receiving device includes a power receiving antenna, rectifying circuit, and receiving-side transmitter. The power transmission control circuit causes the inverter circuit to output preliminary AC power to activate the power receiving device. The receiving-side transmitter transmits, to the power transmitting device, control information of the power receiving device including (i) a coupling coefficient between the power transmitting antenna and the power receiving antenna, (ii) requested voltage of the power receiving device, and (iii) load impedance of the load. The power transmission control circuit determines the control parameter based on the control information by referring to the table, based on the control information, and adjusts the voltage and frequency of the AC power output from the inverter circuit using the control parameter.