An object of the invention is to improve durability of a SOFC system and secure favorable power generation performance during the actual useful service period of the system. In the SOFC system, a fuel gas flow rate to a fuel cell stack is set at F1 at the time of start-up. At a time point when a temperature T of the fuel cell stack reaches a first temperature T1 or higher after the temperature is started to increase, when it is determined that the stack temperature T at the time of the previous system stop is lower than or equal to a predetermined value Tb, the fuel gas flow rate is decreased to F2a (which is less than F1), and when it is determined that the stack temperature T is higher than the predetermined value Tb, the fuel gas flow rate is decreased to F2b (which is less than F2a) to slow the temperature increase rate. Furthermore, when the stack temperature T reaches T2, the fuel gas flow rate is returned to F1 so as to increase the fuel gas flow rate and then the process proceeds to the next process.
