a hydraulic circuitry used in the mechanism as shown this hydraulic control mechanism includes a pump 7 an oil passage 8 for feeding oil from the pump 7 to the hydraulic clutch b and a pressure control mechanism f and a pilot pressure controlled selector valve 9 both disposed in the oil passage 8 the mechanism further includes a rotary type change speed control valve 10 for controlling the three hydraulic cylinders t1 t2 and t3 and a pilot oil passage 11 branched via a needle valve from the oil passage 8 to obtain a pilot pressure for controlling the selector valve 9 the pressure control mechanism f is used for obtaining clutching pressure with predetermined rising characteristics by appropriately restricting a sharp and violent surge in the pressure when the selector valve 9 is operated for clutch engagement each of the hydraulic cylinders t1 t2 and t3 and of the mechanical control mechanism ml for the forward reverse switchover unit c is operatively connected with a check valve 12 together constituting opening closing type check valve means these check valves 12 are provided in an exhaust oil passage 11a branching from the pilot oil passage 11 for the selector valve 9 accordingly for a change speed operation the check valves 12 are opened or closed to vary the pilot pressure to the selector valve 9 thereby to engage or disengage the hydraulic clutch b describing more specifically the check valves 12 are opened or closed depending on whether the control system is currently in the course of a change speed operation or not namely if the system is in a speed changing state the check valves 12 are opened.