A liner 7 is provided with a cylinder portion 14a is formed in parallel to the rotation axis, and also a hydraulic fluid flow path 14c is formed which opens to a cylinder portion 14a to communicate with the interior of the liner serving as a high-pressure chamber and a low pressure chamber at the time of occurrence of the striking torque, and an automatic relief mechanism 14 is provided in which a cylindrical valve element 14b having a cutout portion 14b′ serving as a flow path for hydraulic oil formed on the outer peripheral surface portion is disposed inside the cylinder portion 14a so as to be freely rotatable.