A gas turbine apparatus includes a turbine configured to be driven by a combustion gas from a combustor, an exhaust passage, an extraction line configured to extract a fluid from a combustor casing, a cooling part disposed in the extraction line and configured to cool the fluid, a return line for returning the fluid from the cooling part to an inside of the combustor casing, the return line being connected to the extraction line downstream of the cooling part, a bypass line for introducing the fluid from the cooling part to the exhaust passage by bypassing the turbine, the bypass line branching from the return line, and a communication state switching part for controlling a communication state between the extraction line and the combustor casing via the return line, and a communication state between the extraction line and the exhaust passage via the bypass line.