To provide a steam injection gas turbine generator set capable of recovering heart energy of gas turbine waste heat to the maximum, even if necessary power quantity fluctuates, reducing the necessary fuel gas quantity and enhancing the heat efficiency.

This steam injection gas turbine power generator set is provided with a gas turbine power generator 12, comprising a compressor 12a, combustor 12b, turbine 12c, and generator 11, waste heat recovery boiler 14 generating steam by the waste heat of the turbine, mixing device 16 for mixing fuel gas with the steam, reformer 18 positioned between the gas turbine power generator and the waste heat recovery boiler and reforming fuel gas into reformed gas, including hydrogen, reformed gas storage device 20 for storing the reformed gas, and a reformed gas quantity adjuster 21 for adjusting the reformed gas quantity to be fed from the reformed gas storage device to the combustor.