PURPOSE: To prevent flame failure from occurring without the need of once stopping operation of an operation lever in the course of operation movement even if an operation to slow flame state is rapidly effected, by providing a bypass passage separately from a throttle valve and maintaining flow rate through the bypass passage at a minimum combustion flow rate of a burner.

CONSTITUTION: A bypass passage 36 is provided with a large inner diameter so as to reduce flow resistance, and a throttle 33 is controlled in such a manner that the flow rate of gas bypassed into a supply passage 12 pt the time of closure of a throttle valve 17 becomes a minimum combustion rate. In the case in which a part 3 having an enlarged inner diameter is provided, if the throttle valve 17 is rapidly closed, gas in the vicinity of the throttle valve 17 is pressed back toward the part 3 located upstream so that pressure in the part 3 instantly rises. Gas injection rate is increased and throttling rate is reduced so that flame failure of a burner is surely prevented from occurring.