To eliminate insufficient rotating force and energy loss even if the pressure and displacement are varied.

When a force matching a pressure P in a cylinder oil chamber 4b at the bottom dead center is larger than the spring force of a spring 21, a spool 20 is moved from an intermediate port 18 side toward a drive pressure port 11 side, and communication passages 14b, 14a are cut off in this order. Therefore, the moving stroke of a piston 5 in which the piston 5 reaches the bottom dead center after the communication between the drive pressure port 11 of a valve plate 10 and the cylinder oil chamber 4b is cut off. When a force matching a pressure P in the cylinder oil chamber 4b at the bottom dead center is smaller than the spring force of the spring 21, the spool 20 is moved from the drive pressure port 11 side toward the intermediate port 18 side, and the communication passages 14a, 14b are allowed to communicate with each other in this order. Consequently, the moving stroke of the piston 5 in which the piston 5 reaches the bottom dead center after the communication between the drive pressure port 11 of the valve plate 10 and the cylinder oil chamber 4b is cut off.