Calories of a first mixed gas are predicted by calculations based on the mixed flow rate of a blast furnace gas and the mixed flow rate of a converter gas measured by flow meters, and preset blast furnace gas calories and converter gas calories; the flow rate ratio of the mixed flow rate of a coke oven gas to a gas turbine consumed fuel gas flow rate is calculated based on the predicted calories, set calories, and preset coke oven gas calories; the mixed flow rate required value of the coke oven gas is calculated based on the flow rate ratio and a gas turbine fuel gas requirement signal corresponding to the gas turbine consumed fuel gas flow rate; and the opening of a coke oven gas flow control valve provided in a fuel gas production system is controlled, based on the mixed flow rate required value, to control the mixed flow rate of the coke oven gas.