A suitable method for starting a SOFC system of the present invention leads to improve durability. By combusting the fuel gas supplied from the reformer to the combustion portion in S11 during system start-up, the temperatures of the reformer and the fuel cell stack are increased from room temperature. In S11, an ATR process begins, and the hydrogen-enriched fuel gas is generated by the autothermal reforming reaction. In S12, a cell temperature T is compared to a minimum reduction start temperature T1 of the cell support and, in the case of T ‰¥ (greater than or equal to) T1, the process proceeds to S13. In S13, by setting a mixed molar ratio (O 2 /C) of oxygen in the ATR air supplied to the reformer and hydrocarbon in the raw fuel to 0.3 or more, the hydrogen concentration of the fuel gas is set to 50% or less. In S14, the cell temperature T is compared to a maximum reduction start temperature T2 of the cell support, and in the case of T > (greater than) T2, the process proceeds to step S15. In S15, the t
