A spring retainer includes a locking member which locks one end side of the piston spring, and a rod-shaped member (82) of which one end is in contact with the locking member to restrict a length of the piston spring and the other end is fixed to the piston (11), and the rod-shaped member (82) is installed in a concave portion (140) of the piston (11), and the concave portion (140) has a protruding portion (142), which is extruded by plastic deformation toward a radial direction of the concave portion (140) and is formed of the same member as the opening circumferential edge portion (141), in the vicinity of the opening circumferential edge portion (141).