PURPOSE: To reduce the amount of NOx and enhance the combustion efficiency in a sub-chamber combustion type internal combustion engine, by opening or closing a sucked air amount regulating valve and an EGR regulating valve in accordance with a detection result of the oxygen concentration in the exhaust gas.

CONSTITUTION: At a midway portion of a suction pipe 20 open to a main combustion chamber 19 there are provided a gas mixer 23 equipped with a sucked air flow regulating valve 22 interlocked with operation of an accelerator, and a suction blower 24. An ignition plug 29 is disposed in a sub-chamber 28 facing the main combustion chamber 19, and a sub-chamber fuel supply pipe 30, which branches from a fuel supply pipe 26, is made open to the sub-chamber 28. The suction pipe 20 and an exhaust pipe 21 are connected to each other by an EGR pipe 33 in which an EGR regulating valve 34 is disposed, and an O2 sensor 35 is provided on the exhaust pipe 21, an output signal therefrom being inputted to a control device 36. From the data indicating the relation among an optimum fuel consumption rate, NOx concentration, and excess- of-air rate (oxygen concentration), the amount of EGR and the sucked amount of air are determined on the basis of the measured value of the NOx concentration, to thereby cause the regulating valves 22, 34 to be controlled.