Fuel cell system and control method thereof

A fuel cell system is provided with a fuel cell, a first combustor, a first heating gas return channel and a gas supplier. The fuel cell includes a solid electrolyte cell with an anode and a cathode. The fuel cell generates power by reacting a hydrogen-containing gas and an oxygen-containing gas. The first combustor selectively supplies a heating gas to the cathode of the fuel cell. The first heating gas return channel mixes at least some exhaust gas discharged from the cathode with the heating gas of the first combustor such that a mixed heating gas of the exhaust gas and the heating gas is supplied to the cathode. The gas supplier connected to the first heating gas return channel for supplying the exhaust gas from the cathode to mix with the heating gas of the first combustor.