A solid oxide fuel cell system 1 comprises a fuel cell main body 20 provided with a gas reformer 22 and a fuel cell 20, a reforming gas supply means 12 for supplying a reforming gas to the gas reformer 22, an oxygen-containing gas supply means 13 for supplying an oxygen-containing gas to the gas reformer 22, a water supply means 14 for supplying water or steam to the gas reformer 22 and an ambient temperature sensor 70 for detecting a temperature outside of the solid oxide fuel cell system 1. The system further comprises a control means 15 for controlling during operation of the fuel cell 21 whether, based on a signal from the ambient temperature sensor 70, for carrying out a reforming reaction of the reforming gas with the oxygen-containing gas and/or water in the gas reformer 22, switching between an oxygen-containing gas supply means 13 and water supply means 14 is carried out, or both the supply means 13, 14 are used in combination.