This contactless power transmission device transmits power to a contactless power reception device having a reception-side coil and a reception-side resonator which resonates with the reception-side coil. The contactless power transmission device has a transmission unit including a transmission-side coil, a transmission-side resonator which resonates with the transmission-side coil, and an inverter for feeding power to the transmission-side resonator. The inductance value of the transmission-side coil and/or the capacitance value of the transmission-side resonator is set so that the frequency range in which the phase difference of a primary-side current flowing through the transmission unit relative to a primary-side voltage applied to the transmission unit is zero or above is wider than when the inductance value of the reception-side coil are equal to each other as are the capacitance value of the reception-side resonator and the capacitance value of the transmission-side resonator.