PURPOSE: To enhance the fuel consumption rate by providing an intake side control valve and a scavenge side control valve, respectively in parts of a scavenge air passage on the upper and lower sides of a surge tank located in the scavenge air passage, and by incorporating an auxiliary fuel supply device.

CONSTITUTION: A surge tank 16 is connected in a scavenge air passage 15 for the connection between a crankcase 12 and a cylinder 13. A scavenge air passage 15 is composed of a first scavenge air passage 31 opened to the crankcase 12, and a second scavenge air passage 32 opened to a scavenge air port 26 formed in the cylinder 13. Further, an intake control valve 28 and a scavenge control valve 29 are connected respectively to the scavenge air passages 31, 32, the intake control valve 28 being opened and closed during reciprocation of a piston 14 while the scavenge control valve 29 is opened until the initial stage of the reciprocation of the piston 14. Further, a main injection valve 36 adapted to be operated in a high load and high rotational speed range of the engine is provided in the surge tank 16, and an auxiliary injection valve 37 adapted to be operated in intermediate and low load and middle and low rotational speed ranges is provided in a cylinder head 17.