An oxygen density detecting device for a combustor in which individual adjustment of reference levels for each combustor is not necessary. A memory device stores a reference ion current value from a sensor disposed in the combustor for a predetermined period of time extending from the time instant at which combustion breaks out in the combustor until the ion current has substantially stabilized. The oxygen density of the room air used by the combustor is determined by comparing the ion current value stored in the memory with ion current values which are successively provided by the sensing device. An output signal produced thereby is utilized to control circuit elements including a fuel supplying electromagnetic pump and a combustion air blower used for supplying room air into the combustor.
