PURPOSE: To prevent the flame from propagating into an evaporation chamber even if the ignition of a gas fuel is delayed by keeping the supply volume of the air for attenuation in a high temperature air production section in a volume out of the range of combustion until the ignition of the gas fuel is confirmed.

CONSTITUTION: When an air control valve 47 is ON, an air channel 46b for the introduction of air is throttled and the air passing through it is throttled. On the other hand, unitl a flame detector 49 is ON and the ignition is confirmed, the air control valve 47 is OFF and a channel 46b is kept in the largely open state. Accordingly an excessive volume of air for attenuation is mixed with the gas fuel from an air supplier 46 through the channel 46b and an air introduction chamber 44 until the fuel is ignited. Therefore, in the case in which the ignition is made later than in the normal operation, the combustion is not made, and the propagation of flame to the mixture gas entering an evaporation chamber is prevented. And, when the channel 46b is throttled with the ignition of the gas fuel confirmed by a flame detector 49 and the air control valve 47 is thereby tuned ON, the air for attenuation supplied via the air introduction chamber 44 is regulated to the most suitable value.