A two-shaft gas turbine having a steam injection mechanism, comprising: a compressor having an inlet guide vane, a combustor, turbines, a heat recovery steam generator, a steam valve to control a flow rate of steam to be supplied to the combustor, a fuel valve to control a flow rate of fuel to be supplied to the combustor, characterized in that, the two-shaft gas turbine further comprising: a rotational frequency meter, a flow rate meter to measure the flow rate of the steam to be supplied to the combustor from the heat recovery steam generator, an inlet guide vane position gauge provided to the compressor, a steam valve position gauge, and a control system calculates and outputs command signals to operate openings of the fuel valve, the steam valve, and the inlet guide vane based on measurement signals obtained by the rotational frequency meter, the inlet guide vane position gauge, or the steam valve position gauge, and an output increase command value to the gas turbine.