The invention provides a method for operating a reformed gas production apparatus with which it is possible to achieve a high reforming efficiency while preventing a drop in catalyst activity due to the deposition of carbon. The reformed gas production method uses a reforming catalyst to reform a fuel that contains a hydrocarbon having at least two carbon atoms to produce a reformed gas that includes methane, hydrogen, and carbon monoxide. With this method, a fluid that includes the fuel, at least one of steam and a carbon dioxide-containing gas, and an oxygen-containing gas, is supplied to a reforming reaction region, and with the thermal decomposition index temperature of the fuel, which is determined by the type and the concentration of the hydrocarbons having at least two carbon atoms that make up the fuel, serving as an upper limit temperature of the reforming reaction region, the fluid is brought into contact with the reforming catalyst to produce the reformed gas.