This electric motor is provided with an inner periphery side rotor, an outer periphery side rotor, and a rotating device that can change a relative phase between these rotors by rotating at least one of them about a rotational axis thereof. The rotating device is provided with a first member integrally and rotatably provided to the outer periphery side rotor, and a second member integrally fixed on an inside of the inner periphery side rotor which together with the first member defines a pressure chamber on the inside of the inner periphery side rotor. The rotating device changes a relative phase between the inner periphery side rotor and the outer periphery side rotor by supplying a hydraulic fluid to the pressure chamber. The rotating device is further provided with a linking passage that leaks the hydraulic fluid supplied to the pressure chamber to an outside of the pressure chamber.