An electric power transmission device (130) contactlessly transmits electric power to an electric power receiving device (30). The electric power transmission device (130) includes an inverter (142), a power transmission unit (132) and an electronic control unit (170). The electronic control unit (170) is configured to determine whether a current phase of output current flowing from the inverter (142) to the power transmission unit (132) leads output voltage, and adjust a frequency of the AC power in a direction to reduce a lead angle of the current phase when leading of the current phase relative to the output voltage is detected.
