PURPOSE: To smooth change-over of positive and reverse revolution in such a way that a sleeve which performs positive and reverse revolution of an output shaft is engaged with the neutral position with freedom of engagement and disengagement by means of a detent mechanism, while the sleeve is allowed to escape from the detent mechanism by means of torsion spring force.

CONSTITUTION: An interlocking clutch can be formed by pawls 56, 58 of a sleeve 54 which is slidably coupled in the axial direction to an output shaft 40, and by pawls 60, 62 of a positive sense turning toothed wheel 46 and a reverse sense turning toothed wheel 48 of the reversible change-over gear train 44. A plunger 64 is inserted into the shaft 40, and movement of the plunger 64 is transferred to sleeve 54 through a pin 68 which penetrates the plunger 64, a long hole of the shaft 40 and the sleeve 54. A detent mechanism which is integrated by a ball 74, and a retainer 78 pressed by a spring 76 is provided on the plunger 64, and when the sleeve 54 is at the neutral position, the ball 74 is engaged into a concave part 80 of the shaft 40. The plunger 64 is coupled to a cam follower 84 through a swivel joint. The sleeve 54 is moved by means of torsion spring force of a shifting shaft 100 through the cam follower 84 and the plunger 64.