PURPOSE: To enhance the output characteristics of an engine, by increasing the opening degree of a control valve for a cooling passage upon high engine load operation initiating supercharging operation.

CONSTITUTION: When the supercharged pressure is increased by the operation of an intake-air blower 2 so that the intake-air pressure is changed into its positive value from its negative value, the control voltage of a controller 61 is discharged to the ground, and therefore, a control valve 36 full closes a bypass passage 32 so that all air is introduced into a cooling air passages 34. In the case of that the intake-air pressure is negative, the control valve 36 reciprocally adjusts the flow ratio of intake-air which is branched between the bypass passage 32 (heated by a heat-exchanger 33) and the cooling passage 34 in accordance with the control voltage of the controller 61.