The gas turbine system has: a gas turbine having a compressor, a combustor, and a turbine; a fuel supply mechanism for supplying fuel to the combustor; a composition detection unit for detecting the composition of the fuel; and a controller for controlling the flow rate of the fuel supplied from the fuel supply mechanism to the combustor, on the basis of a function of the exhaust temperature of exhaust gas passing through the turbine and either air pressure of air expelled from the compressor to the combustor or an expansion ratio of the turbine. The controller calculates the specific heat ratio of the combustion gas from the composition of the fuel detected by the composition detection unit, corrects the function on the basis of the calculated specific heat ratio, and controls the flow rate of the fuel on the basis of the corrected function.