PURPOSE: To reduce the absorption energy of a speed change clutch by providing a speed change hydraulic circuit to a speed change switching clutch via a speed change selector valve between a hydraulic pump and a pressure control valve and providing a relief valve between the communicating section of this circuit and the pressure control valve.

CONSTITUTION: When speed change clutches C1-C4 are switched after forward/ rear switching clutches F and R are disengaged, the forward/rear switching clutches F and R are engaged after the speed change clutches C1-C4 have been switched, the switching of the speed change clutches C1-C4 can be completed while the power transmission is cut off. The operating oil pressure of the speed change clutches C1-C4 is set by a relief valve 9, thus the oil pressure increase of the speed change clutches C1-C4 is completed before the operating pressure of the forward/rear clutches F and R is increased. The absorption energy is reduced when the speed change clutches C1-C4 are engaged, the capacity of the speed change clutch can be reduced.