A power-supplying device for wirelessly transmitting alternating current (AC) power to a power-receiving device includes a voltage converter, an inverter circuit connected to the voltage converter, a communication unit configured to receive an output value of a rectification circuit provided in the power-receiving device from the power-receiving device, and a switching control unit configured to control the inverter circuit so that the inverter circuit converts direct current (DC) power into AC power and control whether the voltage converter steps up an input voltage or not based on whether an output value of the voltage converter or the output value of the rectification circuit reaches a specific value.