PURPOSE: To prevent emission of blue and while smoke, by preventing temperature rise of supercharging air by by-passing exhaust gas from a turbine when the temperature of supercharging air supplied to the side of an intake manifold becomes higher than a prescribed value.

CONSTITUTION: When the supercharging pressure reaches a prescribed level, an actuator 19 is operated to open a pipe 15 by a valve 17. Resultantly, the amount of exhayst gas supplied to a turbine 2 is reduced and the speed of rotation of the turbine 2 is lowered to keep the supercharging pressure at a constant level. On the other hand, when the temperature of supercharging air becomes higher than a prescribed level, an electromagnetic solenoid 20 is energized and a valve 18 is opened, so that the exhaust gas by-passes the pipes 15, 16.