Abstract: In an intake air flow increasing device for a combustion engine according to this invention, an air exhausting body is provided forward of an air intake opening, and the air exhausting body is connected to a main compressed air conducting path and an auxiliary compressed air conducting path, while each of the compressed air conducting paths is connected to an air compressing mechanism via a main switching control valve and an auxiliary switching control valve respectively. The main switching control valve is opend and closed according to switching operations of a speed change mechanism when a vehicle is accelerated. The auxiliary switching control valve can work independently from opening/closing operations of the main switching control valve. Thus an enough quantity of intake air required for combustion in the diesel engine can be supplied, in addition, power performance of the combustion engine is ensured regardless of running conditions or heavy things loaded on a vehicle.