PURPOSE: To eliminate the abnormality of combustion light detection in early period by a method wherein the part of a light guiding body, which is exposed to the inside of a combustion chamber, is heated to remove carbon adhered thereto when a period of time from a given angle of crank shaft of the engine to the detection of the combustion light is longer than a predetermined period of time.

CONSTITUTION: A heater 91 as a heating body is wound between the light guiding body 82 and a tubular housing 81 near the tip end 82a of the light guiding body 82 and respective terminals 91a, 91b of the heater are introduced to the outside of the tubular housing 81 under securing the electric insulation thereof. When the detection timing of the combustion light is measured, the period of time from the given angle of the crank shaft to the detection timing is decided whether it is longer than the predetermined period of time or not subsequently. When it is longer than the predetermined period of time, the heating body is heated or heating condition is maintained. According to this method, the carbon grains or the like, which are generated on the tip end of the heater, are burnt out to prevent the combustion light from becoming impossible to catch during idling or the like of the engine.