To provide a power generating machine for a hydraulic system with miniaturized and reduced weight construction and improved durability.

A hydraulic pump 3 and an air cylinder 2 are arranged in series, a piston 4 is inserted into the cylinder head part 7 of the air cylinder 2, and at the cylinder head part 7, a piston control mechanism P for supplying and exhausting compressed air to and from a chamber 6 is provided. A cylindrical hole 33 is opened in the hydraulic pump 3 side wall face of the air cylinder 2, and the portion on the hydraulic pump 3 side of the cylindrical hole 33 is used as a pressurizing chamber 34 for oil pressure. The front end part of the piston rod 5 is held in the cylindrical hole 33, a ring-like damper 31 is arranged at the opening-edge part of the cylindrical hole 33, and the portion adjacent to the piston rod 5 of the damper 31 is projected. The inclined surface of the outer periphery of a projected part 31a of the damper 31 is smoothly curved into an S-shape, and a ring-like washer 35 which is curved along the curve of the inclined surface, is allowed to abut on the inclined surface. A coil spring 36 is arranged between the washer 35 and the piston 4, and the piston 4 is energized by the coil spring 36 in the direction in which the piston departs from the hydraulic pump 3.