PURPOSE: To recover effectively the entire energy of the excess gas generated in a blast furnace with the entire plant by conducting the excess gas which cannot be recovered with an expansion turbine provided to a waste gas system of the blast furnace to a plant for recovering the gas of the other blast furnace.

CONSTITUTION: Expansion turbines 51, 52 are respectively provided to the waste gas systems of two units of blast furnaces 11, 12, thereby forming plants 100, 200 for recovering energy. A piping is branched from one point A of the waste gas system of the furnace 11 in the plant 100 to supply the excess gas of the flow rate that cannot be recovered fully with an expansion turbine 51 to one point B of the waste gas system of the plant 200 through a pressure regulating valve 10. The blast furnace gas discharged from the furnace 12 of the plant 200 and the excess gas from the plant 100 are mixed at said point B and the mixture is guided to an expansion turbine 52 through a speed governing valve 42. A control device is connected to the valve 10 to prevent the backflow of the excess gas.