PURPOSE: To improve fuel consumption in the partial load area of an engine, by causing a supercharger to generate power within a partial load area including a racing condition.

CONSTITUTION: When an engine is in a racing condition, throttle valves 12 and 13 are closed, and after feed air flows in an operation chamber 7 through an air feed passage 11 on the inlet side, the feed air in the operation chamber 7 is adiabatically expanded, and is exhausted in an air feed passage 16 on the outlet side. In a partial load area, the throttle valves 13 and 12 are slightly opened, and feed air flows in the operation chamber 7 through the air feed passage 10 on the inlet side and the air feed passage 9 on the outlet side. This causes a supercharger 1 to generate a work, equivalent to a part shown by slant lines, i.e., power. The power is equivalent to the work which is generated when the feed air, flowing in the operation chamber 7 until its volume reaches a point Vc, is adiabatically expanded from its status with an atmospheric pressure Po to the status with a pressure Pi (negative pressure) in the air feed passage 16 on the outlet side.