PURPOSE: To obtain a precise information of combustion to carry out an appropriate control of combustion, by changing over at least either one of two spark plugs into the one having the function of a flame surface sensor for the other spark plug, at predetermined time intervals.

CONSTITUTION: Spark plugs 1, 2 each having an additional function of a flame sensor, are attached to both left and right end parts of a cylinder head section A, respectively. In this arrangement, the spark plugs 1, 2 are connected to a circuit which is in turn connected to a signal analysis circuit 7 through circuit change-over section 3, 4, and to a circuit which is connected to the signal analysis circuit 7 through coil distributors 5, 6 and an ignition signal processing circuit 8. Further, circuits for alternately and intermittently changing over the spark plugs 1, 2 into flame surface sensors at predetermined intervals are incoporated in the change-over sections 3, 4. Further, in accordance with the flame surface detection output of the spark plugs 1, 2, the signal analysis circuit 7 controls the ignition signal processing circuit 8, a fuel injector 10, etc.