PURPOSE: To keep a mixed gas flow rate constant-control system and a constant calorie control system from interference by a method wherein the blast-furnace gas flow rate which stabilizes after a change is estimated by calculation, the estimate is multiplied by the ratio of coke-oven gas/blast-furnace gas subsequent to the change, and the resultant value is used as a target flow rate of coke-oven gas in a mixed gas-calorie constant-control system.

CONSTITUTION: In a mixed gas flow rate constant-control system the blast- furnace gas flow rate B' which stabilizes after the change is obtained by an equation B'=B(1+a)/(1+a') wherein B represents the flow rate of B gas, (a) represents the ratio of C/B prior to the change to the present flow rate of B gas, and (a') represents the ratio of C/B after the change. What is obtained by this equation is multiplied by (a') and the resultant value is used as a target flow rate of coke-oven gas (value C' or SV) in a mixed gas-calorie constant- control system. As a result, the M gas flow rate constant-control system and the M gas-calorie constant-control system can be kept from mutual interference and the control can be effected smoothly.