Abstract: A damper (D) comprises a screw shaft (1), a screw nut (4) threadably and rotatably engaged with the screw shaft (1), a motor (M) having a rotor (R) connected to the screw nut (4), and a detent mechanism (5) for making the screw shaft (1) unrotatable. Thus, since the screw shaft (1) is made to perform a linear motion and there is provided the detent mechanism (5) for the screw shaft (1), the outside diameter of the damper (D) can be reduced and hence the on-board characteristic of the damper (D) to a vehicle is improved.