To reduce the operational trouble caused by frequent maintenance by preventing the damage to and the deformation of a sealing mechanism, and to drastically improve the productivity of a refining furnace by shortening non-blowing time such as charging of molten iron, tapping of molten steel and removal of skull.

When a transporting treatment from the blast furnace iron tapping to the molten iron pretreatment and decarburization is executed in one reaction vessel, an exhaust gas treating system and an attachable/detachable reaction vessel are used. The molten iron received from the blast furnace is desiliconized after desulfurizing and removing slag, and dephosphorization is executed without removing the slag, and the decarburize-refining is executed by regulating the pressure in the reaction vessel to 1.2-5.0 kgf/cm2.