PURPOSE: To shorten a combustion rise time at ignition time and enable a variation in air temperature caused by an excessive or insufficient of amount of air volume to be adjusted by a method wherein a mixing ratio adjusting means for freely changing a mixing ratio of fuel gas to combustion air to be supplied to a combustion device is arranged.

CONSTITUTION: At ignition time, an air opening 43 is opened to maximum and fuel gas is injected into a combustion device 2 through a gas nozzle 34 and enters a catalyst 25 so as to start the combustion. Almost all the air fed by a blower 1 is passed through the air opening 43 as a supply air. a little bit of combustion air only is supplied from the suction port 26 into the combustion device 2, resulting in richer gas concentration, quickly expanding combustion and a short combustion rise time. After the combustion has progressed, opening of the air opening 143 is decreased to increase an amount of air for catalyst combustion fed through the suction port 26, and then the remaining air is heated by the catalyst 25 as a supply air and finally delivered through a discharging port cover 21. When a remaining capacity of cell 4 varies, a degree of air opening 43 is varied and then volume of combustion air or supply air is adjusted and a hot air temperature is also controlled.