A wireless power supply system (100, 101, 102, 103), comprising: - a power transmission device provided on a ground side and having a power transmission coil, and - a power receiving device provided in a vehicle and having a power receiving coil, wherein - the power transmission coil is configured to transmit electric power to the power receiving coil via a wireless connection, so as to supply the electric power to an electrical load installed in the power receiving device, - the power receiving device includes: - an efficiency calculator configured to calculate a power transmission efficiency (η) based on a transmission power command value (Pbat*) and the electric power supplied to the electrical load; and - a switching controller configured to stop supplying the electric power received by the power receiving coil to the electrical load when the power transmission efficiency (η) falls to a predetermined threshold efficiency or less, and - the power transmission device includes: - a power factor calculator which is configured to calculate a power factor (cosθ) based on a phase difference between a voltage and a current supplied to the power transmission coil; and - a power controller which is configured (i) to control electric power supplied to the power transmission coil according to the transmission power command value (Pbat*), and (ii) to reduce the electric power supplied to the power transmission coil when the phase difference between the voltage and the current supplied to the power transmission coil increases and power factor (cosθ) falls to a predetermined threshold power factor or less, once the electric power supplied to the electrical load has been stopped by the switching controller. - a power transmission device provided on a ground side and having a power transmission coil, and - a power receiving device provided in a vehicle and having a power receiving coil, - the power transmission coil is configured to transmit electric power to the power receiving coil via a wireless connection, so as to supply the electric power to an electrical load installed in the power receiving device, - the power receiving device includes: - an efficiency calculator configured to calculate a power transmission efficiency (η) based on a transmission power command value (Pbat*) and the electric power supplied to the electrical load; and - a switching controller configured to stop supplying the electric power received by the power receiving coil to the electrical load when the power transmission efficiency (η) falls to a predetermined threshold efficiency or less, and - an efficiency calculator configured to calculate a power transmission efficiency (η) based on a transmission power command value (Pbat*) and the electric power supplied to the electrical load; and - a switching controller configured to stop supplying the electric power received by the power receiving coil to the electrical load when the power transmission efficiency (η) falls to a predetermined threshold efficiency or less, and - the power transmission device includes: - a power factor calculator which is configured to calculate a power factor (cosθ) based on a phase difference between a voltage and a current supplied to the power transmission coil; and - a power controller which is configured (i) to control electric power supplied to the power transmission coil according to the transmission power command value (Pbat*), and (ii) to reduce the electric power supplied to the power transmission coil when the phase difference between the voltage and the current supplied to the power transmission coil increases and power factor (cosθ) falls to a predetermined threshold power factor or less, once the electric power supplied to the electrical load has been stopped by the switching controller. - a power factor calculator which is configured to calculate a power factor (cosθ) based on a phase difference between a voltage and a current supplied to the power transmission coil; and - a power controller which is configured (i) to control electric power supplied to the power transmission coil according to the transmission power command value (Pbat*), and (ii) to reduce the electric power supplied to the power transmission coil when the phase difference between the voltage and the current supplied to the power transmission coil increases and power factor (cosθ) falls to a predetermined threshold power factor or less, once the electric power supplied to the electrical load has been stopped by the switching controller. (i) to control electric power supplied to the power transmission coil according to the transmission power command value (Pbat*), and (ii) to reduce the electric power supplied to the power transmission coil when the phase difference between the voltage and the current supplied to the power transmission coil increases and power factor (cosθ) falls to a predetermined threshold power factor or less, once the electric power supplied to the electrical load has been stopped by the switching controller.