PURPOSE: To reduce the weight of a piston return means so as to simplify its structure by sliding and bringing a strike piston into contact with a cylinder such that an annular piston is freely moved in the vertical direction of the strike piston and deciding the tail end of the strike piston during a return process in the step part of an oil chamber adjacent to a piston chamber.

CONSTITUTION: A piston return means includes only one annular piston 8, and this annular piston 8 is slid and brought into contact with a strike piston 1 and a cylinder 3 so as to be freely moved in the vertical direction of the strike piston 1. Further, the play of a motion opposite the striking direction of the annular piston 8 with respect to the strike piston 1 is limited by the contact surface of the strike piston 1, and the tail end position of the strike piston during returning is decided by a cylindrical step part 1c positioned in an oil chamber 12 adjacent to a piston chamber 9. With the approaching of the step part of the strike piston to the backside of the oil chamber 12 facing the piston chamber 9, the oil chamber 12 is connected to a circulation part 17.