A contactless power supply system includes a first power transmission coil, a plug power reception coil, a plug power transmission coil, and a first power reception coil. The first power transmission coil and the plug power reception coil are configured to satisfy a relational equation of (L2a/L1)1/2=G1/K1=N2a/N1, where L1 represents a self-inductance of the first power transmission coil, L2a represents a self-inductance of the plug power reception coil, N1 represents a winding number of the first power transmission coil, N2a represents a winding number of the plug power reception coil, K1 represents a coupling coefficient of the first power transmission coil and the plug power reception coil, and G1 represents a voltage conversion gain of the voltage applied to the plug power transmission coil relative to the voltage applied to the first power transmission coil.