An electric motor driving device that drives an electric motor including a field winding, a rotor and a stator, wherein the rotor and the stator each foam a field pole by passing a field current through the field winding, includes: a power supply device; a converter including a reactor that at least partially serves as the field winding shared with the electric motor, and configured to receive a voltage from the power supply device to carry out voltage conversion between first and second power lines and to pass the field current through the field winding during voltage conversion operation; an inverter configured to convert a direct-current power received from the converter to an alternating-current power for driving the electric motor; and a controller controlling the converter so that a current flows through the field winding in the same direction both during power running and regeneration of the electric motor.