To prevent a steel ball from falling out, and make a clutch mechanism in a screw fastening device small in size, and light in weight.

In a clutch mechanism for a screw fastening device where a sleeve 81 is provided for a driving shaft member 5, a sleeve S2 is provided for an output shaft member 7, and which is provided with a steel ball 15 which can be moved in the radius direction of a through hole 16 provided for the sleeve 81 for either one of these shaft members 5 and 7, and furthermore, can be projected outward from the outer surface of the sleeve S1, and with a means which allows the steel ball 15 to be projected outward from the outer surface of the sleeve S1 in response to the movement of the output shaft, a part of the through hole 16 is made smaller than the outer diameter of the steel ball 15.