A contactless power supply device comprising a power transmission device and a power receiving device to which electrical power is transmitted from the power transmission device in a contactless manner, wherein the power transmission device includes: a transmitter coil which supplies electric power to the power receiving device; a power supply circuit which supplies an alternating current, obtained by stepping up or stepping down the input voltage, to the transmitter coil, a voltage detection circuit which measures the input voltage to the power supply circuit; and a control circuit which controls the degree of stepping up or stepping down the input voltage, by the power supply circuit, with respect to the measured input voltage based on the measured input voltage; and the power receiving device includes: a receiver coil that receives electric power from the transmission device, a resonance circuit having a resonance capacitor which resonates with the receiver coil 21 in accordance with the electric power from the transmission device, and a rectification circuit which rectifies the electric power output from the resonance circuit, wherein the power supply circuit includes: a power factor correction circuit which steps up the input voltage and outputs direct current, and an inverter which converts the direct current that is output from the power factor correction circuit into an alternating current, wherein the control circuit causes the inverter to operate in a full-bridge mode, if the measured input voltage is less than a predetermined voltage, and causes the inverter to operate in a half-bridge mode, if the measured input voltage is greater than the predetermined voltage. a transmitter coil which supplies electric power to the power receiving device; a power supply circuit which supplies an alternating current, obtained by stepping up or stepping down the input voltage, to the transmitter coil, a voltage detection circuit which measures the input voltage to the power supply circuit; and a control circuit which controls the degree of stepping up or stepping down the input voltage, by the power supply circuit, with respect to the measured input voltage based on the measured input voltage; and the power receiving device includes: a receiver coil that receives electric power from the transmission device, a resonance circuit having a resonance capacitor which resonates with the receiver coil 21 in accordance with the electric power from the transmission device, and a rectification circuit which rectifies the electric power output from the resonance circuit, wherein the power supply circuit includes: a power factor correction circuit which steps up the input voltage and outputs direct current, and an inverter which converts the direct current that is output from the power factor correction circuit into an alternating current, a receiver coil that receives electric power from the transmission device, a resonance circuit having a resonance capacitor which resonates with the receiver coil 21 in accordance with the electric power from the transmission device, and a rectification circuit which rectifies the electric power output from the resonance circuit, wherein the power supply circuit includes: a power factor correction circuit which steps up the input voltage and outputs direct current, and an inverter which converts the direct current that is output from the power factor correction circuit into an alternating current, a power factor correction circuit which steps up the input voltage and outputs direct current, and an inverter which converts the direct current that is output from the power factor correction circuit into an alternating current, wherein the control circuit causes the inverter to operate in a full-bridge mode, if the measured input voltage is less than a predetermined voltage, and causes the inverter to operate in a half-bridge mode, if the measured input voltage is greater than the predetermined voltage.