To optimize a control parameter for an internal combustion engine by calculating a criterion through the detection of the amount of ions (ionic current) which are produced within a controlling cylinder immediately after ignition, and correcting the control parameter if the result of comparison of the criterion with a reference value shows lowering of the output of the internal combustion engine or worsening of the combustion state.

During engine operation, combusting an air-fuel mixture with sparks produced by the ignition plugs 8a to 8d of cylinders results in the generation of ions in a combustion chamber, causing an ionic current (i) to flow with the charging voltage of a capacitor 9 as a power supply. Then the ionic current (i) is converted to voltage by a current-voltage converting circuit 12, the current- voltage conversion gain is switched by a gain switching circuit 13 according to the voltage level of an ionic current detection signal Ei, and a gain switching signal G is input to an ECU 2. The ECU 2, after converting the peak value of the signal Ei from analog to digital form along with the signal G, multiplies the signals together to produce a criterion and, if the difference of the criterion from an averaged reference peak value is not less than a permissible value, corrects ignition timing to advance it.