A pressure leading path in a impact type tool is branched halfway through a bypass passage for connecting a high pressure chamber H with a low pressure chamber (L). The branch includes a first fixed aperture leading to the high pressure chamber (H). The branch also includes a second fixed aperture leading to the low pressure chamber (H). The pressure leading path is connected to a primary side of a relief valve. An automatic shutoff mechanism is operated by hydraulic operating fluid relieved from the secondary side of relief valve. The automatic shutoff mechanism cuts off the air supply to the air motor driving the tool, and thus automatically shuts off the tool, when the pressure in the pressure leading path exceeds a predetermined value. The present invention permits control of pulsed torque with high precision in a simple construction.