PURPOSE: To aim at improvement of the power output in a high-speed operating range by providing a heat exchanger for cooling the supercharged air, and then controlling the cooling performance in response to the accelerator operating amount and the supercharged air temperature.

CONSTITUTION: There is provided a water-cooling type intercooler 4 for cooling the supercharged air from a supercharger 2. Between a heat exchanger 5 and a sub-radiator 7 for the intercooler 4 is circulated the cooling water by means of a water pump 6. In response to the accelerator operating amount and the supercharged air temperature, the supply voltage of a water pump motor is varied so that the quantity of water supplied to the heat exchanger 5 is varied. Hereby, even in a high-speed operating range, the rise of the intake air temperature can be stopped so that the power output can be improved.