PURPOSE: To remove the influence from uneven light intensity caused by combustion by using the initial peak value of a light-emitting signal to correspond to the initial combustion peak caused by ignition and firing of inflammable air- fuel mixture in a combustion chamber.

CONSTITUTION: Microwaves of a fixed frequency are generated a microwave oscillator 1, while a microwave unit I is provided to detect 3 the received microwaves and convert them into a low frequency signal. Then, a probe II is provided to be composed of a microwave sensor to radiate microwaves in a combustion chamber and receive the reflected waves and a light-sensor to detect the light-emitting state caused by the combustion chamber electrically. Meanwhile, a light signal processing unit 7 is provided to detect 6 the peak of the microwave signal outputted from the microwave unit I and the initial peak value in the light-emitting signal outputted from the light sensor of the probe II in each cycle for calculating 9 the time difference between the peak middle point of the microwave and the time when the light-emitting signal reaches to an intensity of a fixed ratio of the initial peak value. This construction permits to detect the timing of combustion precisely.