Abstract: A tightening apparatus includes a drive motor and a bit are coupled to each other by engagement of a clutch mechanism, the engagement thereof being maintained by a force of a spring one end of which is coupled to the clutch mechanism and the drive motor is released from engagement with the bit when the tightening torque of the bit exceeds a value corresponding to the force of the spring. The tightening apparatus comprises a cylinder and a plunger encased in the cylinder for holding the other end of the spring. The plunger is movable by a drive mechanism such that the force of the spring is varied, whereby the tightening force to be applied to the bit is adjustable in accordance with the position of the plunger as adjusted by the drive mechanism.