PURPOSE: To eliminate radial projections to obtain a simplified construction of a device by arranging a driving cylinder of an engaging means consisting of a stopper claw and a receiving groove in the same direction with a main shaft.

CONSTITUTION: Power of extremely low frequency and low voltage is supplied to a motor M. Then, the piston rod 18a of a second air-operated cylinder 18 is operated in its extending direction in state where the motor M is driven at extremely low speed. Since the shaft output torque is small, and its speed is extremely low, the engagement of a stopper claw 20 with a receiving groove 21 is performed smoothly without large impact. In this case, the stopping position of the main shaft 4 is regulated sufficiently even when the engaging strength is small. When the action of the stopper claw 20 is confirmed by a detecting switch 23, the power supply to the motor M is stopped or connected, and at this position, replacement of a cutting tool 10 is performed.