Patent attributes
A prime mover includes a stator fixed to a housing, and a rotor rotatable relative to the stator. The prime mover is capable of outputting torque from the rotor by being supplied with electric power. A speed reducer is capable of reducing the torque of the prime mover and outputting the reduced torque. The speed reducer includes a sun gear rotatable integrally with the rotor, and planetary gears capable of revolving in a circumferential direction of the sun gear while rotating in a state of meshing with the sun gear. A gear width of the sun gear is set such that a length in the axial direction by which the sun gear and the planetary gear are overlapped is smaller than a gear width of the planetary gear.