In a non-contact power feeding system which is formed with a power transmission device that includes a power transmission-side resonant circuit including a power transmission-side coil and a power reception device that includes a power reception-side resonant circuit including a power reception-side coil and which can transmit and receive power by a magnetic field resonance method, the control circuit of the power transmission device supplies, before the power transmission, a predetermined determination alternating current voltage to the power transmission-side resonant circuit so as to detect the amplitude of a current flowing through the power transmission-side coil at that time. The control circuit controls the performance of the power transmission based on the current amplitude detection value.