A wireless power supply system (100, 101, 102, 103), - comprising a power transmission device and having a power transmission coil and a power receiving device 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, - wherein the power receiving device includes: - a first efficiency calculator configured to calculate a first 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 first efficiency (η) falls to a predetermined first threshold efficiency or less, and - wherein the power transmission device includes: - a second efficiency calculator configured to calculate a second efficiency (cosθ) based on a phase difference between a voltage and a current supplied to the power transmission coil; and - a power controller configured to control electric power supplied to the power transmission coil according to the transmission power command value (Pbat*), and to reduce the electric power supplied to the power transmission coil if the second efficiency (cosθ) falls to a predetermined second threshold efficiency or less or if the electric power supplied to the electrical load is stopped by the switching controller. - comprising a power transmission device and having a power transmission coil and a power receiving device 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, - wherein the power receiving device includes: - a first efficiency calculator configured to calculate a first 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 first efficiency (η) falls to a predetermined first threshold efficiency or less, and - a first efficiency calculator configured to calculate a first 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 first efficiency (η) falls to a predetermined first threshold efficiency or less, and - wherein the power transmission device includes: - a second efficiency calculator configured to calculate a second efficiency (cosθ) based on a phase difference between a voltage and a current supplied to the power transmission coil; and - a power controller configured to control electric power supplied to the power transmission coil according to the transmission power command value (Pbat*), and to reduce the electric power supplied to the power transmission coil if the second efficiency (cosθ) falls to a predetermined second threshold efficiency or less or if the electric power supplied to the electrical load is stopped by the switching controller. - a second efficiency calculator configured to calculate a second efficiency (cosθ) based on a phase difference between a voltage and a current supplied to the power transmission coil; and - a power controller configured to control electric power supplied to the power transmission coil according to the transmission power command value (Pbat*), and to reduce the electric power supplied to the power transmission coil if the second efficiency (cosθ) falls to a predetermined second threshold efficiency or less or if the electric power supplied to the electrical load is stopped by the switching controller.