PURPOSE: To detect the combustion in a gas chamber due to backfire and interrupt supply of fuel instantaneously, by providing a flame detecting circuit for detecting rectification and conduction of current between a flame detecting electrode and the wall of the gas chamber which is caused by flame produced in the gas chamber.

CONSTITUTION: By providing the flame detecting electrode 12 in the gas chamber 3, there is constituted a flame detecting circuit for detecting rectification and conduction of current between the flame detecting electrode 12 and the wall of gas chamber 3 which is caused by flame produced in the gas chamber 3. By employing such an arrangement capable of interrupting fuel supply immediately by the function of the flame detecting circuit, if combustion is caused in the gas chamber 3 due to backfire, presence of flame in the gas chamber 3 can be detected directly and immediately by the flame detecting electrode 12. By thus interrupting fuel supply immediately, it is enabled to prevent burning of the gas chamber 3 and other parts of the combustor and hence occurrence of fire.