PURPOSE: To shut off surely transmition torque with set torque and always enable a power tool to be operated with stable torque by providing the drive side cam means tending to axially part from each other when a drive force from a drive unit acts.

CONSTITUTION: After set torque is reached and a drive force acts from a drive unit 1, cam means 3a, 8a acting to part axially part from each other are separated. Thus, a clutch ring 8 is moved toward a bit end against a force of a torque spring 15 to move forcibly a clutch sleeve 9 in the same direction through a stopper ring 7 so that clutch claws 9c, 17c are disengaged from each other and the drive force is not transmitted to the bit 18. Then, when the clutch sleeve 9 moves toward the bit end, a parting ball 12 rides over a ridge portion between circumferential grooves 9', 9" of the clutch sleeve 9 and are urged to the position of root diameter of the circumferential groove 9' to disengage the clutch claws 8h, 9h from each other.