PURPOSE: To prevent danger by a method wherein a reaction is stopped by fusing the aft part of the nozzle to cool it and a reverse flow path is clogged by fused metal even in case the fused metal flows reversely and reacts with oxidizing agent gas in the nozzle upon an abnormal reaction.

CONSTITUTION: A nozzle throat section, a valve seat section or a valve rod, second tube and a nozzle tip, constituting the nozzle, are formed by a fireproof metal and this nozzle section operates normally without being damaged by fusing. When it happens that the valve and the seat sections are faulty and the fused metal is flowed reversely, then, is reacted with the oxidizing agent gas in the nozzle, high chrome copper, forming the aft part of the nozzle or a spring and the first tube, is fused upon the abnormal reaction and whereby it is cooled to stop the reaction anh clogg the reverse flow path. The nozzle assembly and the burner are constituted so as to be intercepted by a component having nozzle throat holes so that the reaction at the aft part of the nozzle will never affect to the burner. Molybdenum alloys such as Mo0.5Ti alloy, TZM alloy, TZC alloy, WZM alloy, Mo50Re alloy or the like are preferable as the fireproof metal.