PURPOSE: To aim at protecting an engine, by lowering the supercharged pressure when the cooling function of an intercooler drops to prevent the occurrence of knocking.

CONSTITUTION: If the difference between the temperature T1 detected by a first temperature sensor 10 and the temperature T2 detected by a second temperature sensor 11, that is, the temperature difference across an intercooler 9 is below, for example, 10°C, which shows that the intercooler 12 is in an abnormal condition, and therefore the supercharged pressure is lowered to turn on a warning lamp 18. In the case of that the temperature difference (T1-T2) is, for example, from 10 to 35°C, the intercooler 9 operates in the normal condition. When the temperature difference (T1-T2) is above, for example, 35°C the cooling capacity of the intercooler 9 is sufficient so that the supercharged pressure is raised.