A non-contact power feeding system comprising a power transmission device and a power reception device, transmission and reception of electric power being possible therebetween by magnetic field resonance, wherein the power transmission device includes: a transmission-side resonance circuit including a transmission-side coil for transmitting the electric power; a power transmission circuit which is adapted to feed an alternating-current voltage to the transmission-side resonance circuit to generate a magnetic field in the transmission-side coil; and a transmission-side control circuit which is adapted to operate in, as an operation mode thereof, one of a plurality of modes including a first mode and a second mode; the power reception device includes: a reception-side resonance circuit including a reception-side coil for receiving the electric power; a changing/short-circuiting circuit which can change a resonance frequency of the reception-side resonance circuit from a resonance frequency during the power reception or short-circuit the reception-side coil; and a reception-side control circuit which is adapted to operate in, as an operation mode thereof, one of a plurality of modes including the first mode and the second mode, characterized in that the power transmission device includes a sensing circuit which is adapted to sense an amplitude of a current passing in the transmission-side coil, wherein in the first mode, the reception-side control circuit, according to a signal from the power transmission device by communication and by using the changing/short-circuiting circuit, is adapted to change the resonance frequency of the reception-side resonance circuit, or short-circuit the reception-side coil, for a predetermined time, and then cancels the change or the short-circuiting, and the transmission-side control circuit, during a period in which the resonance frequency of the reception-side resonance circuit is changed or the reception-side coil is short-circuited and prior to the power transmission, controls the power transmission circuit such that a predetermined test magnetic field is generated in the transmission-side coil, then, based on first evaluation data representing a sensed amplitude value obtained from the sensing circuit while the test magnetic field is being generated in the first mode, is adapted to judge whether or not execution of the power transmission is permissible, and, after judging that execution of the power transmission is permissible, controls the power transmission circuit such that a power transmission magnetic field stronger than the test magnetic field is generated in the transmission-side coil to carry out the power transmission, and in the second mode, the reception-side control circuit, by using the changing/short-circuiting circuit, is adapted to maintain the change of the resonance frequency of the reception-side resonance circuit or the short-circuiting of the reception-side coil, and the transmission-side control circuit is adapted to control the power transmission circuit such that the test magnetic field is generated continuously in the transmission-side coil, and acquires second evaluation data representing a sensed amplitude value obtained from the sensing circuit while the test magnetic field is being generated in the second mode. a transmission-side resonance circuit including a transmission-side coil for transmitting the electric power; a power transmission circuit which is adapted to feed an alternating-current voltage to the transmission-side resonance circuit to generate a magnetic field in the transmission-side coil; and a transmission-side control circuit which is adapted to operate in, as an operation mode thereof, one of a plurality of modes including a first mode and a second mode; a reception-side resonance circuit including a reception-side coil for receiving the electric power; a changing/short-circuiting circuit which can change a resonance frequency of the reception-side resonance circuit from a resonance frequency during the power reception or short-circuit the reception-side coil; and a reception-side control circuit which is adapted to operate in, as an operation mode thereof, one of a plurality of modes including the first mode and the second mode,