Abstract: An impact tool including a cylinder, a piston slidably mounted in the cylinder and a valve for selectively opening and closing oil feed channels to move the piston up and down. A cylindrical member is slidably mounted on the piston around its large-diameter portion or thereunder. Its bottom end defines the top end of a lower chamber formed in the cylinder. The cylindrical member is adapted to descend together with the piston. But when the piston strikes the object and rebounds upwardly, the cylindrical member keeps descending away from the piston owing to the inertia force. Thus the volume of the lower chamber is kept from increasing excessively after the piston has struck the object. This will prevent cavitation in the lower chamber. When thereafter the lower chamber communicates with the oil feed port and pressure oil is fed thereinto, the cylindrical member will be pushed up toward the large-diameter portion under oil pressure in the lower chamber.