PURPOSE: To detect both flames of an igniter and a main burner positively by installing an objective for a torch having the wide field of view for detecting the flame of the igniter and an objective for a burner having the narrow field of view to an optical probe.

CONSTITUTION: An objective 5 for a torch having the wide field of view for detecting the flame 16 of an igniter 14 and an objective 20 for a burner having the narrow field of view for detecting the flame 17 of a main turner 15 are juxtaposed to true front end section 2a of an optical probe body 2. The flame 16 of the igniter 14 is detected mainly by the objective 5 for the torch, and the flame 17 of the main burner 15 is detected principally by the objective 20 for the burner. Since the objective 5 for the torch has the wide field of view 18, the small flame or the igniter 16 is not displaced from the field of view 18 even when the small flame swings, and the flame 16 can be detected positively. Since the objective 20 for the burner has the narrow field of view 19, on the other hand, the flicker of a remote flame by the combustion of the main burner 15 can be detected.