PURPOSE: To provide a combustion chamber of an engine in which mixed burnt gas is moved immediately to a main conbustion chamber and also NOx in the exhaust gas can be reduced in the case of an engine provided with an auxiliary combustion chamber in which fuel is used.

CONSTITUTION: A control valve 4 which opens and closes the injection hole 32 of an auxiliary conbustion chamber 31 at least at the latter half of a compression stroke and at the former half of an explosion stroke is opened to connect a main combustion chamber 22 to the auxiliary combustion chamber 31 and, at least at an exhaust strike, the control valve 4 is closed to seperate the main combustion chamber 22 from the auxiliary combustion chamber 31 so as to retain exhaust gas in the auxiliary combustion chamber 31. Thus exhaust gas re-circulation can be performed.