Rotational motion of a shaft member rotationally driven by a motor is converted by a screw member into translational motion of a pressing member. An outer periphery of a cap member is brought into slide contact with a first cylindrical part of the pressing member so that the cap member is relatively movable in an axial direction of the pressing member and is relatively unrotatable about its axis. An inner periphery of the cap member is brought into slide contact with a second cylindrical part of the shaft member so that the cap member is relatively movable in an axial direction of the shaft member and is relatively rotatable about its axis. The first cylindrical part, the second cylindrical part, and the cap member partition a storage chamber connected to one end of the screw member, and a lubricant for lubricating the screw member is filled inside the storage chamber.