PURPOSE: To make an air-fuel ratio detectable in succession, by setting up a couple of electrodes to be impressed with high voltage inside a combustion chamber, while processing an external current detecting value according to the output of an O2 sensor, in time of detecting the external current based on a move toward the electrode of charged particles on fire.

CONSTITUTION: A couple of electrodes 4 and 5 to be impressed with high voltage from a DC power source 6 are arranged face-to-face each other in an engine combustion chamber 1 while particles inside the combustion chamber, now under combustion, are ionized on fire which shows electrical characteristics. According to polarity each has, these charged particles move to each electrode and thereby they are detected by a current detection circuit 7 as an external current. This detected value turns to the maximum value in and around a theoretical air-fuel ratio and is reversed in the variable direction so that a process circuit 9 outputs the external current value to an output terminal 13a or 13b via gates 10 and 11 opened or closed by an 02 sensor 8; the air-fuel ratio can be detected in succession from 13a in time of being rich and from 13b in time of being lean respectively.