PURPOSE: To improve the safety and reliability of the combustion equipment by a method wherein a photo-cell is located facing to flame and a combustion controlling device is controlled by electric signals in accordance with the quantity of light of the flame at the captioned indoor open type equipment.

CONSTITUTION: When the oxygen deficience state is brought about, the flame turns into yellow flame and emits a large quantity of light. As a result, the resistance of CdS of the photo-cell 14 reduces remarkably enough for a detecting device 13 to detect the reduction. The operations of a blower and the like are controlled by the detection signal of the detecting device 13 through a controlling unit. In this case, the reduction of the resistance of CdS is so large that the voltage drop across CdS is large even under the condition that the reduction of ion current of a flame rod 11 is small. Accordingly, the oxygen deficience state can be surely detected even in the case that a highly stable burner is employed, resulting in improving the safety and reliability of the equipment.