Abstract: A method of operating a combined plant which is composed of a gas turbine system including a compressor into which air is introduced through an inlet guide vane, a combustor in which a fuel is burnt with the air compressed by the compressor, and a turbine into which the combustion gas is introduced to generate an output power, a waste-heat recovery boiler which generates steam by the heat possessed by the combustion exhaust gas from the gas turbine system and a steam turbine driven by the steam generated by the waste heat recovery boiler. The opening of the inlet guide vane of the gas turbine system is controlled in accordance with the state of the waste-heat recovery boiler or the steam turbine, such as the temperature of the steam introduced into the steam turbine, thermal stress generated in the steam turbine, temperature of a metallic part of the steam turbine and so forth.