A retaining unit 8 is constituted by a first retainer 25 and a second retainer 26. The first retainer 25 and the second retainer 26 are mounted, respectively, on a bleeder screw 7 of a cylinder body 11 and a projecting portion of a piston rod 19. While the piston rod 19 is being forced in a cylinder bore 13, first and second engagement portions 25d, 26e are brought into engagement with each other, and the piston rod 19 is retained in a retracted state. The engaged state of the first and second engagement portions 25d, 26e is released by removing the first retainer 25 from the bleeder screw 7, whereby the piston rod 19 is allowed to extend.