PURPOSE: To enable reliable detection of ignition without being influenced by an ambient temperature, by a method wherein ignition is detected through detection of a change in an output voltage produced by oxygen concentration.

CONSTITUTION: A sensor 1 is made of a material such as zirconia, and is an oxygen concentration battery type sensor which generates an electromotive force occasioned by a change in oxygen concentration, and detects ignition and incomplete combustion. A least minimum value detector part 24 of a microcomputer 23 in an ignition detecting circuit 21 inputs the output power voltage of a zirconia sensor, and decides whether the inputted voltage value is an extreme value. When it is minimum value, the detector part detects a least minimum value through comparison of a present minimum value Z (2) with a least minimum value Z (0) of preceding one and before. An ignition judging part 25 deducts the least minimum value Z (0) from a value Z (3) of a zirconia sensor, and when the result exceeds a predetermined ignition detecting voltage V, it is judged that ignition is made.