A striking cylinder 6 is disposed within a body 1. A main air chamber 7 for reserving compressed air therein is disposed at an outside of the striking cylinder 6. A cylinder cap 8 for covering the striking cylinder 6 and the main air chamber 7 is fixed at an upper portion of the body 1. A lower portion of a cylindrical head valve 13, which opens and closes the main air chamber 7 and the striking cylinder 6 and is configured in a manner that upper and lower external diameters thereof are the same, is disposed at an outside of the striking cylinder 6. A cylindrical inner wall 10 and a cylindrical outer wall 11 are integrally formed at a lower surface of the cylinder cap 8. A piston stop 12 for defining a top dead center of a striking piston 5 is provided at an inside of the cylindrical inner wall 10. The head valve 13 is slidably provided between the cylindrical inner wall 10 and the cylindrical outer wall 11.