A housing includes an accommodation space for a prime mover and a speed reducer. A rotational translation unit includes a rotation portion that rotates relative to the housing upon receiving torque outputted from the speed reducer, and a translation portion that moves relative to the housing in an axial direction according to rotation of the rotation portion. The accommodation space is between the rotation portion and the housing. A clutch in a clutch space allows or interrupts transmission of torque between a first transmission portion and a second transmission portion. The rotation portion is between the clutch space and the accommodation space. A sealing member has an annular shape in contact with the rotation portion, and maintains an air-tight or liquid-tight state between the accommodation space and the clutch space.