In a fuel cell system of the present invention, a reformed gas generated in a reformer (R1) being activated is supplied to a fuel cell stack (F1), and an off-gas discharged from the fuel cell stack (F1) is supplied to a heat supply device (B2) provided for a reformer (R2) being deactivated. By activating at least one reformer (Rn), all of a plurality of reformers (Rn) can be warmed-up. Therefore, energy consumption in a standby state can be suppressed, and the fuel cell system can be started-up quickly in emergencies. The reformed gas may be supplied to the heat supply device (B2) instead of the off-gas.