PURPOSE: To dephosphorize molten iron to a target content of phosphorus within a target treating time in a process for dephosphorizing the molten iron with oxygen and lime by calculating a dephosphorizing volocity constant from a dephosphorizing rate and a treating time and controlling the energy for agitation of the molten metal, the supplying speed of lime and the temp. of the molten metal.

CONSTITUTION: Prior to steel making and refining of molten iron by decarburization, the P contained therein is oxidized to P2O5 and is combined with CaO, whereby the P is slagged off and removed. In this case a dephosphorizing velocity constant k'p is calculated by the equation (1) in the equation (I) from the dephosphorizing time and the contents of the P before and after the dephosphorization, and the dephosphorization is so performed as to satisfy either one of values i or ω CaO eff in the equation (2) basing on the actually measured temp. of the molten metal (T) and either a value for the energy for agitation (i) of the molten metal or the set value of the supplying rate of CaO (ω CaO eff) in the equation (2) in the equation (I) using thereby the value of k'p, whereby the P in the molten iron is removed down to the target content of P within the target time. Here the supplying speed of CaO in the equation (2) is calculated from the equations (3), (4) in the equation (II).