PURPOSE: To efficiently remove phosphorus in molten iron by adding converter slag ordinarily produced with blow-refining as sub-raw material into the molten iron to be subjected to dephosphorizing treatment.

CONSTITUTION: The molten iron tapped from a blast furnace is supplied to pre-treating process composed of desiliconization, desulfurization and dephosphorization on a casting floor. In the above dephosphorizing process, the converter slag 34 is charged to the molten iron 31. The converter slag 34 is one after supplying to the dephosphorizing treatment in the blow-refining. Oxygen gas is blow into the molten iron 31 from a second lance 33, and by burning carbon in molten iron 31, heat is generated. Dephosphorizing agent is blown into the molten iron 31 from a first lance 32, and the molten iron 31 and converter slag 34 are stirred and mixed. Then the phosphorus in the molten iron 31 is allowed to react with CaO in the converter slag 34 and the phosphorizing agent to become stable phosphorus oxide through calcium phosphate and the phosphorus oxide is transferred into the dephosphorizing slag.