A two-shaft gas turbine having a steam injection mechanism, comprising: a compressor having an inlet guide vane to control a flow rate of air to be introduced, a combustor to mix the air compressed by the compressor with fuel and combust the air with the fuel to generate a combustion gas , a turbine having a high pressure turbine driven by the combustion gas generated in the combustor and a low pressure turbine disposed on a downstream side of the high pressure turbine , a heat recovery steam generator to generate steam using an exhaust gas discharged from the low pressure turbine as a heat source, a steam system to supply the combustor with the steam generated by the heat recovery steam generator , a fuel valve provided to a fuel system to supply the combustor with the fuel and control a flow rate of the fuel to be supplied to the combustor , a steam valve provided to the steam system and control a flow rate of the steam to be supplied from the heat recovery steam generator to the combustor through the steam system , a load to be driven by the turbine, a compressor shaft to connect the high pressure turbine with the compressor to each other, and a low pressure turbine shaft to connect the low pressure turbine with the load to each other, wherein the high pressure turbine is configured to drive the compressor , and the low pressure turbine is configured to drive the load , characterized in that the two-shaft gas turbine having the steam injection mechanism further comprising: at least one of a rotational frequency meter to measure a rotational frequency of the compressor ; an inlet guide vane position gauge to measure opening of the inlet guide vane provided to the compressor ; a steam valve position gauge to measure opening of the steam valve provided to the steam system ; and a flow rate meter to measure the flow rate of the steam generated by the heat recovery steam generator and to be supplied to the combustor ; and a control system configured to calculate and output command signals to operate openings of the fuel valve , the steam valve , and the inlet guide vane , respectively, based on measurement signals obtained by one of the rotational frequency meter , the inlet guide vane position gauge , the steam valve position gauge and the flow rate meter , and based on an output increase command value to the gas turbine. at least one of a rotational frequency meter to measure a rotational frequency of the compressor ; an inlet guide vane position gauge to measure opening of the inlet guide vane provided to the compressor ; a steam valve position gauge to measure opening of the steam valve provided to the steam system ; and a flow rate meter to measure the flow rate of the steam generated by the heat recovery steam generator and to be supplied to the combustor ; and a control system configured to calculate and output command signals to operate openings of the fuel valve , the steam valve , and the inlet guide vane , respectively, based on measurement signals obtained by one of the rotational frequency meter , the inlet guide vane position gauge , the steam valve position gauge and the flow rate meter , and based on an output increase command value to the gas turbine. <EOS>