PURPOSE: To perform smooth operation during rise of burning, by a method wherein hydrocarbon gas is divided into two lots, the one lot for heating a catalyst oxide mat is connected to one piping, while the other lot for heating a forming catalyst and for modifying reaction is connected to the other piping.

CONSTITUTION: One of the two lots, into which natural gas is divided by an electromagnetic valve 12, is mixed with the air in a piping 15 to feed it in a fuel modifying device 10. Within the fuel modifying device 10, flame is formed on a burner port plate 18 with the aid of an ignition plug 19 to preheat a reforming catalyst 17. Meanwhile, after natural gas, which is branched by the electromagnetic valve 12 and is fed to a piping 7, is mixed, within a duct 6, with the air which is fed from an air intake port 8 through a piping 9, flame is formed on a burner port plate 4 to preheat a catalyst oxide mat 3. This reduces a time required for preheating, and allows exhaust gas, produced after reforming catalyst is preheated, to be utilized for preheating of the catalyst oxide mat.