PURPOSE: To abolish a hydraulic mechanism so as to eliminate the leakage of oil by providing a transmitting shaft behind a driven shaft, forward and reverse gears and forward and reverse clutches in the front part on the transmitting shaft and an operating mechanism for interrupting both the clutches.

CONSTITUTION: A driven shaft 3 in a prime mover side and a transmitting shaft 5, for transmission to a speed change mechanism, behind the driven shaft are provided. Forward and reverse gears 6, 7 are rotatably provided before the transmitting shaft 5, and forward and reverse clutches 8, 9 are provided between both the gears 6, 7. Rotation of the driven shaft 3 is transmitted to the transmitting shaft 5 from gears 21, 25, forward gear 6 and the forward clutch 8 or from the gears 21, 25, transmitting shaft 27, gears 26, 31, reverse gear 7 and the reverse clutch 9. Action of this selection is operated by a mechanically operating mechanism 10. Accordingly, structure is simplified and no leakage of oil is generated by eliminating a hydraulic mechanism.