NOVELTY - A fuel and an oxidizing gas are passed through the upper and lower catalyst layers (R1, R2) of a main catalyst (Ra) and a sub catalyst (Rb), in order, during starting of a reformer. The reformer is switched to steady operation, by passing the fuel and water vapor through main and sub catalysts, in order, or bypassing the upper catalyst and passing through the remaining catalysts, when oxidizing gas supply is stopped.    USE - For reforming fuel, such as natural gas, methane, or liquefied petroleum gas, to produce hydrogen-rich reformed gas used in fuel cell for electric power generation.    ADVANTAGE - Enables producing heat by the reaction between the fuel and the oxidizing gas, during starting of the reformer, and utilizing the heat for heating the lower catalyst layer and the sub catalyst, without the use of a separate heat source. Since water vapor is passed through the catalysts, after the catalysts are heated, the water vapor is heated efficiently so that the reformed gas is produced stably and continuously. Enables producing water vapor by heating the water vapor generator using the heat of the catalysts.    DETAILED DESCRIPTION - The upper catalyst layer is preheated before the starting of the reformer. The ratio of the fuel and the oxidizing gas supplied to the upper catalyst layer, during starting, is adjusted corresponding to the temperature raise of the upper catalyst layer which comprises the precious metal catalyst or a non-precious metal catalyst containing noble metals. The sub catalyst comprises a non-precious metal catalyst.    An INDEPENDENT CLAIM is also included for reformer with a detector to detect whether the upper catalyst layer has reached a preset temperature. A controller (14) enables to starting the reformer, when the detected temperature matches with the preset temperature.    DESCRIPTION OF DRAWING(S) - The figure shows a partial longitudinal sectional view of the reformer. (Drawing includes non-English language text).    controller (14)    upper and lower catalyst layers (R1,R2)    main catalyst (Ra)    sub catalyst (Rb)