A gas turbine includes a compressor and a combustor; a SOFC having a cathode and an anode; a first compression air supply line supplying compression air to the combustor; a second compression air supply line supplying compression air to the cathode; an exhaust air supply line supplying exhaust air discharged from the cathode to the combustor; a first fuel gas supply line supplying a fuel gas to the combustor; a second fuel gas supply line supplying a fuel gas to the anode; a fuel gas supply ratio change unit capable of changing a supply ratio of the fuel gas supplied to the combustor and the fuel gas supplied to the anode; an exhaust fuel gas supply line supplying an exhaust fuel gas discharged from the anode to the combustor; and a controller performing open-close control of the control valves and according to an operation state of the SOFC.