PURPOSE: To reduce the generation of black smoke and noxious exhaust gas constituent by a method wherein a main combustion chamber and an auxiliary combustion chamber are communicated with each other through an through hole which is open and closed in timing and further communicated through an opening having a smaller diameter than that of the through hole.

CONSTITUTION: The main combustion chamber 4 and the auxiliary combustion chamber 6 are communicated with each other through the through hole 8 via the auxiliary combustion chamber valve 9 which is open and closed by means of a valve mechanism. Furthermore, the opening 14, which is smaller than the through hole in diameter, is bored to communicate the main combustion chamber 4 with the auxiliary combustion chamber 6. When a piston 3 reaches to the point A from bottom dead center, the fuel injection from a fuel injection nozzle 7 into the auxiliary combustion chamber 6 is started and at the same time the auxiliary combustion chamber valve 9 closes the through hole 8. After that, the high temperature highly compressed air is jetted through the opening 14 in the auxiliary combustion chamber 6 forcibly, resulting in generating violent vortex within the auxiliary combustion chamber 6. After that, when the piston reaches near top dead center, the auxiliar combustion chamber valve 9 is open.