The fuel cell system according to the present invention comprises a reformer 12 for receiving a hydrocarbon fuel supply and generating a hydrogen-containing reformed gas by making use of a reforming reaction; a fuel cell assembly 14 for generating power after causing an anode to receive the reformed gas and causing a cathode to receive an oxygen-containing cathode gas; cathode off-gas supply flow path 20 for supplying a cathode off-gas, which is discharged from the cathode, to the reformer 12; and bypass flow path 24 for bypassing the cathode and directly supplying the cathode gas to the reformer 12 at the time of system warm-up.