PURPOSE: To obtain a high pump efficiency, by controlling a variable means according to a parameter relating to the engine speed.

CONSTITUTION: At the time of low-load operation of an engine when the opening of a throttle valve 16 is small, operation of an air pump 20 is stopped through uncoupling of an electromagnetic clutch 23, so that the engine is operated only by the intake air drawn naturally from a main intake port 7. When the opening of the throttle valve 16 becomes greater than a predetermined value, the clutch 23 is coupled and the air pump 20 is set into operation. In this stage, supercharging air is supplied from the main intake port 7 by the function of the air pump 20 is addition to the naturally drawn air. Further, when the speed is rotation of a pulley 22 is high, a second supercharging passage 19 is displaced downward, so that the length of a supercharging intake passage on the upstream side of the air pump 20 is shortened. On the other hand, when the speed of rotation of the pulley 22 is low, the length of the supercharging intake passage 19 is increased, so that an effective inertial effect of intake air is obtained.