PURPOSE:To remarkably reduce the frequency of control actions by estimating the changing tendency of the future molten iron temp. from the furnace heat index mutation quantity and load lowering velocity mutation quantity at present and controlling the furnace heat based on the estimated result. CONSTITUTION:The furnace heat index mutation quantity at present is estimated from the furnace heat index reference level corresponding to the desired molten iron temp., the load lowering velocity mutation quantity at present from the load lowering velocity reference level at the furnace temp. corresponding to the desired molten iron temp., and the molten iron temp. after a specified time from the influence time of both mutation quantities on the molten iron temp. The furnace heat is controlled based on the estimated result to reduce the variation of the molten iron temp. By this method, the molten iron temp. is stabilized, and the cooling down of the blast furnace is prevented.