PURPOSE: To compact a clutch structure and stabilize its function by arranging a changeover collar so that it can glide to the connecting position against the reaction, installing a blocking ball capable of transferring in the radial direction, and allowing to move in the axial direction within the original positioning.

CONSTITUTION: A coupling ball 13 hops out in the radial direction from a gap 39 in a journal 2 by a starting surface 38 of a changeover collar 4 and intrudes into a ring-shaped groove 41. A blocking ball 12 is pressed to the groove 41 by a blocking sleeve 3 and intrudes into the groove. When it is secured by the blocking sleeve 3, the blocking sleeve 3 receives the changeover ring thrust force immediately, and as a result, only a grasped winding 27 works against the reaction of a spring 19 to secure an adjusting ring 8 in its original position. When the grasped winding 27 is set off, the spring force works through a pressurizing ring 11 to press the blocking sleeve 3 to the right, and the blocking ball 12 is pushed out by the force of a moment of rotation, and the changeover sleeve 4 is dislocated to the right.