A contactless electric power transmission device includes a power transmission assembly, a first temperature sensor, a second temperature sensor, and an electronic control unit. The first temperature sensor is configured to detect a temperature of an inverter. The second temperature sensor is configured to detect a temperature of a resonance circuit. The electronic control unit is configured to adjust the frequency by controlling the inverter. The electronic control unit is configured to perform first control when the temperature of the inverter is higher than the temperature of the resonance circuit, and perform second control when the temperature of the resonance circuit is higher than the temperature of the inverter. The first control includes control for adjusting the frequency so as to reduce output current of the inverter. The second control includes control for adjusting the frequency so as to reduce current flowing through the resonance circuit.