To provide a technique in which excess rising of slag temperature in a molten iron pretreatment in a converter, and dephosphorizing reaction stagnant and rephosphorizing caused by lowering of dephosphorizing performance, is prevented and the reaction is efficiently progressed.

Secondary combustion ratio in the furnace is made to <12% while keeping a state of high slag-making ratio at 70% liquid-phase ratio in the slag composition, under state of ≤1,400°C molten iron temperature and thus, the lowering of the dephosphorizing performance caused by the heating of the slag with the radiation from the secondary combustion flame, is eliminated.