To raise the temperature of a raw fuel gas /reformed gas system without supplying only steam to a fuel cell at a preheating operation time, and allow no influence on reforming instrument catalyzer.

A fuel electrode inlet shutoff valve 20 and a fuel electrode inlet atmospheric relief valve 21 are provided on the upstream side of the fuel electrode 1a of an arranged pipe 10. In the case of the execution of steam temperature raising at a starting time, the fuel electrode inlet shutoff valve 20 is closed and the fuel electrode inlet atmospheric relief valve 21 is opened by a control means 30. Therefore, in the case of temperature raising no steam is supplied to the fuel electrode 1a, and the deterioration of cell characteristics is prevented. Since the sufficient steam temperature raising is conducted, the system path of the arranged pipe 10 is sufficiently heated, no condensation of the steam is allowed, and a sufficient steam amount can be supplied together with raw fuel gas.