PURPOSE: To use low calory gas as fuel for a conventional gas turbine system by combining a tubine system connected to an air compressr and a turbine system connected to a low calory gas compressor thereby approximately equalizing the weight flow rates passing the compressor and turbine of each system.

CONSTITUTION: An cir compressor 6 compresses the air supplied via a line "a" and distribute it to combustors 10 and 13 via an air preheater 9 while a blast furnace gas as a low calory gas is supplied to a blast furnace gas compressor 7 via a line "d" and is distributed to the combustors 10 and 13 via a blast furnace gas preheater 10. The heated gas from the combustor 10 is supplied to the turbine 4 while the heated gas from the combustor 13 is supplied to the turbine 5. The rotating speed control of the turbines 4 and 5 is done through the control of flow regulator valves 19 and 23 corresponding to the signals from rotating speed detectors 32 and 33.