To surely perform flame detection even for combustion flame with low illuminance such as blue flame combustion.

A light sensor 5 is set in an optional position of a support member 4 through which a combustion nozzle 1 extending along a central axis S, the support member being opened to combustion flame side while surrounding the circumference of the combustion nozzle. A number of condenser lenses 6 are arranged in circumferential positions concentric to the central axis S on the opening side of the support member so that their light receiving axes Xs are parallel to the central axis X. Light condensed by each condenser lens is guided to the optical sensor by an optical fiber 7 and made incident on the optical sensor.