A wireless power-supplying system comprising a power-transmitting device and a power-receiving device, wherein the power-transmitting device includes a power-transmitting coil to which AC power of a certain frequency is input from a power supply and a controller which controls the frequency, the power-receiving device includes a power-receiving coil which is magnetically coupled with the power-transmitting coil with a certain coupling coefficient and a power-receiving side series element connected to the power-receiving coil in series and having an imaginary impedance jZ S2i, the imaginary impedance is defined so that impedance when a power-receiving side is seen from the power supply is independent of the coupling coefficient when the frequency and the coupling coefficient are predetermined values, and the controller changes the frequency according to at least one of power-transmitting status information of the power-transmitting device and power-receiving status information of the power-receiving device when the coupling coefficient changes, wherein a fixed load is connected to the power-receiving device as a power-supply target, characterised in that the imaginary impedance of the element is defined to satisfy Equation when the frequency and the coupling coefficient of the power supply are predetermined values as initial value, V 2 V 1 = L 2 L 1 where L 1 is self-inductance of the power-transmitting coil, L 2 is self-inductance of the power-receiving coil, V 1 is a voltage across the power-transmitting coil, and V 2 is a voltage across the power-receiving coil, and wherein a load is connected to the power-receiving device as a power-supply target, the power-receiving status information is load information which is one of power, a current, and a voltage input to the load, and the controller changes the frequency so that the load information approaches a desired value of the load information which is the one of the power, the current, and the voltage. wherein the power-transmitting device includes a power-transmitting coil to which AC power of a certain frequency is input from a power supply and a controller which controls the frequency, the power-receiving device includes a power-receiving coil which is magnetically coupled with the power-transmitting coil with a certain coupling coefficient and a power-receiving side series element connected to the power-receiving coil in series and having an imaginary impedance jZ S2i, the imaginary impedance is defined so that impedance when a power-receiving side is seen from the power supply is independent of the coupling coefficient when the frequency and the coupling coefficient are predetermined values, and the controller changes the frequency according to at least one of power-transmitting status information of the power-transmitting device and power-receiving status information of the power-receiving device when the coupling coefficient changes, wherein a fixed load is connected to the power-receiving device as a power-supply target, characterised in that the imaginary impedance of the element is defined to satisfy Equation when the frequency and the coupling coefficient of the power supply are predetermined values as initial value, V 2 V 1 = L 2 L 1 where L 1 is self-inductance of the power-transmitting coil, L 2 is self-inductance of the power-receiving coil, V 1 is a voltage across the power-transmitting coil, and V 2 is a voltage across the power-receiving coil, and wherein a load is connected to the power-receiving device as a power-supply target, the power-receiving status information is load information which is one of power, a current, and a voltage input to the load, and the controller changes the frequency so that the load information approaches a desired value of the load information which is the one of the power, the current, and the voltage.