PURPOSE: To prevent gear screaking while improving operating feeling by pressing a ball retained in a plunger with roughly symmetrical tapered surfaces from both sides thereof to be engaged into a recess provided on the inner surface of the power shaft.

CONSTITUTION: As a shift lever 84 is turned to the forward rotation position F, a cam 86 is twisted counterclockwise and a torsional spring force applied on a shift shaft 82 reaches the set load of a detent mechanism to release a ball 60 from a recess 70. At the same time, a cam flower 72, a plunger 54 and a sleeve 44 are moved to a gear 32 for forward rotation with the a snap action by a torsional spring force stored in the shift shaft 82 until pawls 46 and 50 engage together. By turning the lever 84 to the reverse position R, the sleeve 44 is meshed with a gear 34 for reverse rotation to reverse a screw. The detent mechanism is released both in the forward and reverse direction from the neutral position by a roughly equal set load so as to make the load almost equal in the meshing of the sleeve 44 with the gears 32 and 34.