To relax an impact at a movement limit of a piston with a simple structure.

A cushion concave part 22R and a passive flat plane 23R are respectively formed at an end cover 21R and a front cover, a cushion plunger 16R fitted in the cushion concave part 22R and a driving plane 15R are respectively formed at end and front surfaces of a piston 12, the cushion concave part 22R and the cushion plunger 16R are respectively formed in a concave shape and a convex shape whose diameters are reduced toward top parts to form similar figures fitted in each other at an interval, a communication groove 25R is formed which communicates a rear port 14R with the cushion concave part 22R, and a displacement liquid amount by the cushion plunger 16R in the cushion concave part 22R is gradually increased and an opening area of an exhaust gap formed between the cushion concave part 22R and the cushion plunger 16R is gradually reduced as the top part of the cushion plunger 16R is fitted in the cushion concave part 22R.