PURPOSE: To prevent the occurrence of the kick back phenomenon of a clutch pedal, by a method wherein, in a vehicle provided with the power shift device of a subtransmission gear and a clutch booster, a check valve is installed between the actuator of the power shift device and the clutch booster.

CONSTITUTION: In a vehicle provided with a main transmission gear 4 shifted and controlled by a shift lever 11 and an auxiliary transmission gear 5 shifted and controlled by a power shift device equipped with an actuator 27, the vehicle has a clutch booster 35, sharing the driving source (negative pressure source) 36 of an actuator 27 with the actuator, to increase the pedalling force of a clutch pedal 9 to exert it on a master cylinder 15. The actuator 27 shifts and controls the auxiliary transmission gear 5 with electromagnetic valves 23 and 24 switched corresponding to operation of a gear position select switch 19. In which case, a check valve 41 is aligned in series to an orifice 48 between a vacuum pump 37 and the clutch booster 35, and a vacuum tank 46 is disposed between the check valve 41 and the booster 35.