PURPOSE: To prevent the lowering of the output of the Diesel engine at the time of speed reduction during the high speed operation of the engine by a method wherein a vehicle speed detecting switch is attached to the intake throttle valve control device in series with an acceleration switch by the ON-OFF operations of which the control device is operated.

CONSTITUTION: A switching device comprising a rod 17 and a diaphragm 16 is attached to an intake throttle valve 14 of a Diesel engine provided with a turbocharger and the negative pressure from a negative pressure tank 24 is supplied to the diaphragm 16 through changeover valves 21 and 23. Further, the vehicle speed detecting switch 29 is provided in a drive circuit of a relay switch 27 for controlling the changeover valve 21, in series with the acceleration switch 28. Thus, when the engine is operated at a speed higher than a predetermined speed, the accelerator is returned to its original position to enable a gear change operation to be made so that even if the switch 28 is turned ON, the switch 29 is kept ON to keep the valve 21 ON and the atmospheric air is introduced into diaphragm chambers 18 and 19 to thereby keep the throttle valve 14 opened to a sufficient degree.