To provide an air/fuel ratio detecting method of a gas combustor which considers a more safe aspect and is more enhanced in detection capacity, and to provide an air/fuel ratio detection sensor thereof, and the gas combustor.

A fuel cell, which reaches a predetermined high-temperature state to generate electricity, is arranged in the vicinity of the combustion flame of the gas burner of the gas combustor and a combustion gas is brought into contact with the fuel electrode of the fuel cell while the electromotive force of the fuel cell caused by the contact of combustion air with an air electrode is detected and the air/fuel ratio of the gas burner is detected on the basis of the value of the detected electromotive force. Further, the air/fuel ratio detection sensor used therein and the gas combustor equipped with the air/fuel ratio detection sensor are also disclosed.