A piston coupled to a piston rod is inserted in a cylinder, and an outer cylinder is disposed around the cylinder so as to define a reservoir therebetween. A separator tube is disposed around the cylinder so as to define an annular passage therebetween. The piston rod can be locked and unlocked by closing and opening an electromagnetic open/close valve so as to block and allow a flow of hydraulic fluid through a flow passage between the annular passage and the reservoir. The separator tube has a greater thickness. O-rings are disposed so as to provide seals between the separator tube and the cylinder. The separator tube extends to positions such that the ends of the separator tube overlap a base valve and a rod guide, thereby holding the respective ends of the cylinder. Due to this configuration, it is possible to prevent deformation of the cylinder and the separator tube due to an increase in the hydraulic pressure.