PURPOSE: To expand the merit of pre-treatment of molten iron, to improve the accuracy of starting time of casting and to reduce the unexpected cast change or recast request by executing a continuous production control from a refining with a converter to a continuous casting to execute the straightening of product physical distribution.

CONSTITUTION: After executing de-Si treatment of molten iron tapped from a blast furnace, dephosphorizing agent is added to execute de-P and de-S, and after such pre-treatment, the molten iron is charged into the converter and refined by blowing the oxygen and successively, after executing degassing treatment with the secondary refining, the continuous casting is executed. In the producing method of a low phosphorus steel, the completing time of pre- treatment of the molten iron is prediction-corrected based on the dividing timing of the blast furnace and the actual result after de-Si just near the time. By this method, the molten iron quantity supplied into the converter is obtd. Further, the molten iron quantity used according to the converter schedule going back from the continuous casting timing, is obtd. The starting timing of the continuous casting is decided so that the supplied molten iron quantity matches to the used molten iron quantity. Consequently, the development of the change of casting speed and the cast stop caused by the molten iron is eliminated to obtain the improvement of the productivity and the yield of the low P steel production.