A non-contact power supply system comprising: a power supply device for transmitting high frequency power; and a load device which is adapted to contactlessly receive the high frequency power transmitted from the power supply device by electromagnetic induction to supply the high frequency power to a load, wherein the power supply device includes a power transmission unit having a primary power coil for power transmission and an inverter circuit for supplying high frequency current to the primary power coil; an inquiry unit having a primary signal coil for receiving a signal from the load device and an oscillation circuit, the primary signal coil being connected between output terminals of the oscillation circuit; a signal detection unit for detecting the signal received by the primary signal coil; and a control unit for controlling the power transmission unit according to the signal detected by the signal detection unit, wherein the load device includes the load; a power reception unit having a secondary power coil for power reception which is magnetically coupled to the primary power coil and a power conversion unit which is adapted to convert the high frequency power induced in the secondary power coil into power for the load; a secondary signal coil which is magnetically coupled to the primary signal coil; and a response unit which is operated by an electromotive force induced in the secondary signal coil to transmit the signal from the secondary signal coil, wherein the control unit of the power supply device does not execute the power transmission from the power transmission unit when the signal detection unit does not detect the signal and executes the power transmission from the power transmission unit when the signal detection unit detects the signal, and characterized in that the oscillation circuit of the inquiry unit is adapted to oscillate at a frequency higher than that of the inverter circuit, and the response unit has a power circuit which is adapted to produce operation power from a voltage induced in the secondary signal coil and a modulation circuit which is operated by the operation power produced by the power circuit to output a modulation signal of a frequency lower than that of the inverter circuit to the secondary signal coil. a power supply device for transmitting high frequency power; and a load device which is adapted to contactlessly receive the high frequency power transmitted from the power supply device by electromagnetic induction to supply the high frequency power to a load, wherein the power supply device includes a power transmission unit having a primary power coil for power transmission and an inverter circuit for supplying high frequency current to the primary power coil; an inquiry unit having a primary signal coil for receiving a signal from the load device and an oscillation circuit, the primary signal coil being connected between output terminals of the oscillation circuit; a signal detection unit for detecting the signal received by the primary signal coil; and a control unit for controlling the power transmission unit according to the signal detected by the signal detection unit, wherein the load device includes the load; a power reception unit having a secondary power coil for power reception which is magnetically coupled to the primary power coil and a power conversion unit which is adapted to convert the high frequency power induced in the secondary power coil into power for the load; a secondary signal coil which is magnetically coupled to the primary signal coil; and a response unit which is operated by an electromotive force induced in the secondary signal coil to transmit the signal from the secondary signal coil, wherein the control unit of the power supply device does not execute the power transmission from the power transmission unit when the signal detection unit does not detect the signal and executes the power transmission from the power transmission unit when the signal detection unit detects the signal, and characterized in that the oscillation circuit of the inquiry unit is adapted to oscillate at a frequency higher than that of the inverter circuit, and the response unit has a power circuit which is adapted to produce operation power from a voltage induced in the secondary signal coil and a modulation circuit which is operated by the operation power produced by the power circuit to output a modulation signal of a frequency lower than that of the inverter circuit to the secondary signal coil.