PURPOSE: To hasten self-ignition combustion by shifting each piston stroke of cylinders relatively to each other by connecting each scavenge passage of a large cross sectional area, which is opened to a combustion chamber at its one end, with each long and slender scavenge passage at the other end, and by joining the long and slender scavenge passages with each other at a point whereby they are divided into two equal length.

CONSTITUTION: Intake air is divided into two flows: one is for NO.1 induction system A where a mixture or air is supplied form an entire operating area of an engine, and the other one is for NO.2 induction system B where an engine load is identical to a predetermined load or more and only air is supplied for an operating area. And NO.1 supercharger 15, an auxiliary fuel injector 16 and the like are provided to NO.1 induction system A for feeding a pressurized mixture and the like to a joint section 17 which joins two long and slender passages 18 provided to cylinders. In this case, the piston stroke of each of plural cylinders #1 and #2 is relatively shifted in phase by 180° to each other, furthermore, the scavenge passages 18 are connected with a pair of scavenge passages, the cross sectional area of which is larger, at points equally spaced from the joint section 17. On the other hand, NO.2 throttle valve 42, NO.2 mechanical supercharger 60 and the like are provided to NO.2 induction system B, and the supercharger 60 is connected with NO.2 scavenge passage 40 at the downstream of the supercharger.