PURPOSE: To prevent an overspeed by detecting a load at the time of starting a vehicle and the like from the opening of an accelerator, applying a preliminary air pressure to an air clutch to carry out feed forward control, and obtaining a necessary and sufficient half clutch condition at the time of reaching a set engine speed.

CONSTITUTION: As the signal of the opening sensor of an accelerator 1 is transmitted to a computer 4 through an interface 5, the computer 4 transmits a signal to an electromagnetic proportional valve 15 through an interface 14, to open a passage 18 and apply a preliminary air pressure to an air clutch 16. Then, as an engine speed detected by an electromagnetic pickup 2 is transmitted to the computer 4 through an interface 3, the computer 4 transmits a signal to the valve 15 through the interface 14, to open the passage 18 and apply an air pressure in which a preliminary pressure based on the accelerator opening is added to a pressure proportionate to the engine speed, to the air clutch 16 to obtain a necessary and sufficient half clutch condition. Accordingly, unnecessary rise in engine speed at the time of reaching a set engine speed can be eliminated.