To miniaturize pistons while securing sealing performance between the mutual pistons in a two piston type.

A piston part 20 is composed of a lower piston 28 and an upper piston 56. The upper piston 56 is pressed in a recessed part 30 of the lower piston 28. Thus, sealing performance between mutual pistons is secured, and since a seal member such as an O ring can be abolished, the pistong part 20 can be miniaturized in the shaft direction and the radial direction. Since an outside diameter of a peripheral wall part 28B of the lower piston 28 changes by press-in, a band 78 is installed in an outer peripheral part of a main body part 28A uninfluenced by the press-in.