PURPOSE: To improve the characteristics of an engine by having the compressed air of a compressor act in the spring side chamber of a waste gate actuator to delay the opening motion of the waste gate so as to increase the supercharged pressure when an engine runs at a low speed.

CONSTITUTION: When an engine is started, the signal 20 is sent to a controller 18. The controller 18 then turns on a solenoid valve 15 so that compressed air from a compressor 3 acts in the compressing chamber 8 of a waste gate actuator 7 and also acts in a spring-side chamber 9 via a bypass passage 13. The opening motion of the waste gate is therefore delayed and the pressure in the actuator 7 becomes the preset value Pa1 of a pressure sensor 16 so that the pressure sensor 16 sends a signal 17 to the controller 18. When a revolutional signal 20 from an engine speed sensor 19 reaches Ne1, the controller 18 turns off the solenoid valve 14. The waste gate is then opened, having a part of exhaust gas bypass to the atmosphere side.