To eliminate the need of altering layout of auxiliary parts, and modifying or newly providing a cylinder block in mounting by mounting a Roots type air compressor driven by engine power on the upper surface of a cylinder head.

In the outer periphery of a head cover 6, a case body 12 of a supercharger 11 formed by a Roots type air compressor driven by an engine is fixed to a cylinder head 1. The rotation of a crankshaft by engine starting is transmitted at the double speed through a belt, a driving pulley and a driving shaft to a rotor 14 constituting the Roots type air compressor, and another rotor 14' is driven at the constant speed in the reverse direction through the driving shaft, a gear and a driven shaft. Fresh air introduced into a housing 13 through an intake port is compressed by the rotors 14, 14', and then introduced into a surge tank 26 through an intake pipe 24. The high pressure fresh air flows into an intake port 2 of each cylinder through an intake branch pipe 24a and is mixed with fuel injected from an injector 30 in the midway.