PURPOSE: To prevent incomplete combustion due to decting shortage of oxygen, by amplifying a variation of an ionic current according to radiant heat which is radiated from a flame and is decided its quantity by a temperature of flame.

CONSTITUTION: When oxygen density within a room and a surface temperature of a combustion plate 7 are remarkably reduced during combustion, quantity of radiant heat to be detected by a light detecting element 17 is sharply reduced and a resistance value of the light detecting element 17 is increased steeply. Accordingly, if values of resistances 19 and 20 are set appropriately, terminal voltage of the resistance 19 shows remarkable variation through synergism with the light detecting element 17, even when an ionic current have shown only a scanty variation. With this, a combustion state can be confirmed accurately, and before air is in short supply of the oxygen density, indoor oxygen density can be detected easily and incomplete combustion can be prevented easily from occurring.