NOVELTY - Temperature conditions (I) of catalyst layer (4) which modifies fuel (CC) of volume smaller than that during completion of starting, and temperature conditions (II) of catalyst layer which can modify fuel of flow volume during completion of starting, are determined. The temperature of catalyst layer is increased while measuring temperature. The fuel flow at which the catalyst layer is capable of modifying, is determined by comparing the measured temperature with the conditions. When the determined flow volume exceeds present value, fuel is supplied.    USE - Starting method of fuel-cell system used in cogeneration systems and electric-power generation systems used for mobile components.    ADVANTAGE - The method prevents anode from being degraded by oxidation, since modification is carried out from an early stage.    DETAILED DESCRIPTION - A fuel-cell system consisting of a modifier (3) having a modification-catalyst layer which modifies a hydrocarbon group fuel to produce a hydrogen-containing gas, and a high-temperature-form fuel cell (6) which produces electric power using the obtained hydrogen-containing gas, is started as follows. Temperature conditions (I) of catalyst layer which can modify fuel of flow volume smaller than fuel flow volume during completion of starting, and temperature conditions (II) of catalyst layer which can modify fuel of flow volume during completion of starting, are determined. The temperature of catalyst layer is increased while measuring the temperature. The flow volume of fuel at which the catalyst layer is capable of modifying the gas when the temperature is measured is determined by comparing the measured temperature with the temperature conditions. When the determined flow volume exceeds present value of fuel flow volume, fuel of determined flow volume is supplied to the catalyst layer, and the obtained reformed gas (AA) is supplied to the anode of fuel cells. The determination of fuel flow volume and supply of fuel are repeated in turns until the fuel feed rate to the catalyst layer becomes the flow volume during completion of starting. An INDEPENDENT CLAIM is included for fuel-cell system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of solid oxide fuel cell system. (Drawing includes non-English language text)    Modifier (3)    Modification-catalyst layer (4)    Fuel cell (6)    Reformed gas (AA)    Hydrocarbon group fuel (CC)