A relay apparatus in a wireless power transmission system includes a relay power reception antenna that receives power transmission alternating current power from a power transmission power transmission antenna, a relay rectifier that converts the power transmission alternating current power into relay direct current power, a relay inverter circuit that converts the relay direct current power into relay alternating current power, and a relay power transmission antenna that wirelessly transmits the relay alternating current power. When transmitting data to the power transmission apparatus through amplitude modulation, the relay apparatus varies amplitude of voltage of the power transmission alternating current power received by the relay power reception antenna between a first amplitude and a second amplitude and performs control for eliminating a difference between a third amplitude of the relay alternating current power and a fourth amplitude of the relay alternating current power.