PURPOSE: To permit the detection of the flame even under the combustion a of low air-fuel ratio by a method wherein the attenuating rate of a light volume, permeating through the light beams permeating window of the head of the flame detector, is estimated as the function of variables of a combustion parameter and a combustion continueing time and a correcting operator for the light volume, which is outputting detecting light volume signal after correcting, is equipped in the detector.

CONSTITUTION: The light of flame, introduced to a flame detector head 5 after permeating through a quartz window at the tip end of the detector, is sent to a light sensor 6 to convert it into an electric signal, subsequetly, is amplified by an amplifier 7, thereafter, is sent to the light volume correcting operator 8 to effect A/D conversion. A process signal 10, consisting of the air-fuel ratio and the combustion continueing time as the combustion parameters, is inputted into the light volume correcting operator 8, the attenuating rate of the quartz window permeating light, corresponding to the input, is memorized beforehand, the A/D conversion signal is corrected by the amount of attenuation computed based on the attenuating rate and the A/D conversion signal is indicated by an outputter 9 as a flame information which is not affected by the adhesion of soot.