PURPOSE: To improve a response of an engine in a low load range, by detouring a compressor in an intake passage, providing a bypass passage, opening a bypass throttle valve in accordance with opening of a throttle valve in the intake passage and providing said bypass throttle valve in said bypass passage.

CONSTITUTION: At starting or the like of an engine, when the first throttle valve 5 in an intake passage 4 is placed at a low opening, a movable contact 16 of an idle switch 14 on a shaft of the throttle valve is contacted to the side of a fixed contact 17 to turn ON a transistor 22 in a controller 13. This causes a solenoid 15 to be excited, and the second throttle valve 12, in a bypass passage detouring a compressor 6 of a supercharger, is opened. Then intake air, flowing through an air flow meter 7, is supercharged to the engine 1 by the compressor 6, simultaneously the air partially flows in the bypass passage 11 and is returned to the upstream side of the compressor 6. In this action, a response of the engine can be improved when the throttle valve 5 is reopened.