An electric power receiving device comprising: a resonance circuit including a resonance capacitor and a resonance coil acting as a receiving antenna, the electric power receiving device being operable to receive electric power in a non-contact manner with the use of resonant coupling of the resonance circuit, a rectifier circuit operable to rectify an AC voltage corresponding to the electric power received by the resonance circuit to obtain a DC output voltage; and an adjustment unit operable to monitor the output voltage of the rectifier circuit and to adjust impedance of the resonance circuit to keep the output voltage from exceeding a target voltage wherein, in receiving the electric power, the electric power receiving device is configured such that the reception electric power received by the resonance circuit is monitored, and a resonance frequency of the resonance circuit is controlled to keep the reception electric power from exceeding a target electric power level, wherein when the reception electric power is not exceeding the target electric power level, the impedance of the resonance circuit is adjusted so as to match the resonance frequency of the resonance circuit with an electric power transmission frequency, and wherein when the reception electric power is exceeding the target electric power level, the impedance of the resonance circuit is adjusted so as to shift the resonance frequency away from the electric power transmission frequency, wherein the capacitance value of the resonance circuit is changed by the adjustment unit, characterized in that the adjustment unit comprises: a differential amplifier circuit operable to generate a control voltage so as to reduce a difference of the target voltage and the output voltage, wherein the resonance circuit comprises: a parallel resonance unit configured with the resonance coil and the resonance capacitor coupled in parallel; and a first capacitor and a first variable resistance circuit coupled in series between one end of the parallel resonance unit and a reference node supplied with a fixed voltage, wherein the first variable resistance circuit is configured to change a value of resistance on the basis of the control voltage, and wherein the resonance circuit further comprises: a second capacitor and a second variable resistance circuit coupled in series between the other end of the parallel resonance unit and the reference node, wherein the second variable resistance circuit is configured to change a value of resistance on the basis of the control voltage, and wherein the rectifier circuit is configured to receive the electric power via the one end and the other end. a resonance circuit including a resonance capacitor and a resonance coil acting as a receiving antenna, the electric power receiving device being operable to receive electric power in a non-contact manner with the use of resonant coupling of the resonance circuit, a rectifier circuit operable to rectify an AC voltage corresponding to the electric power received by the resonance circuit to obtain a DC output voltage; and an adjustment unit operable to monitor the output voltage of the rectifier circuit and to adjust impedance of the resonance circuit to keep the output voltage from exceeding a target voltage wherein, in receiving the electric power, the electric power receiving device is configured such that the reception electric power received by the resonance circuit is monitored, and a resonance frequency of the resonance circuit is controlled to keep the reception electric power from exceeding a target electric power level, wherein when the reception electric power is not exceeding the target electric power level, the impedance of the resonance circuit is adjusted so as to match the resonance frequency of the resonance circuit with an electric power transmission frequency, and wherein when the reception electric power is exceeding the target electric power level, the impedance of the resonance circuit is adjusted so as to shift the resonance frequency away from the electric power transmission frequency, wherein the capacitance value of the resonance circuit is changed by the adjustment unit, characterized in that the adjustment unit comprises: a differential amplifier circuit operable to generate a control voltage so as to reduce a difference of the target voltage and the output voltage, wherein the resonance circuit comprises: a parallel resonance unit configured with the resonance coil and the resonance capacitor coupled in parallel; and a first capacitor and a first variable resistance circuit coupled in series between one end of the parallel resonance unit and a reference node supplied with a fixed voltage, a parallel resonance unit configured with the resonance coil and the resonance capacitor coupled in parallel; and a first capacitor and a first variable resistance circuit coupled in series between one end of the parallel resonance unit and a reference node supplied with a fixed voltage, wherein the first variable resistance circuit is configured to change a value of resistance on the basis of the control voltage, and wherein the resonance circuit further comprises: a second capacitor and a second variable resistance circuit coupled in series between the other end of the parallel resonance unit and the reference node, wherein the second variable resistance circuit is configured to change a value of resistance on the basis of the control voltage, and wherein the rectifier circuit is configured to receive the electric power via the one end and the other end.