PURPOSE: To make it possible to obtain sufficient supercharging pressure in the whole range of engine speed by supercharging compressed air to a suction side by means of a Roots blower whose rotation is driven by engine torque and a centrifugal blower driven by a timing gear.

CONSTITUTION: A Roots blower 39 is driven by an engine through a pulley 31, and a centrifugal blower 63 is increasingly driven through the timing gear 27 of the Roots blower 39. This supplies a suction manifold with compressed air and supercharges fresh air to a cylinder to improve a volumetric efficiency. At this moment in the range where engine speed is low and the supercharging pressure caused by the centrifugal blower 63 is insufficient, the supercharging pressure caused by the Roots blower 39 compensates the shortage thereof, and in the range where engine speed is high, the centrifugal blower 63 supplies sufficient air while both blowers 39 and 63 perform supercharging. Therefore, sufficient engine torque from a low speed range to high speed range can be taken.