PURPOSE: To efficiently and economically desiliconize molten pig iron preliminarily in a converter, by using as a desiliconizing agent slag which has been formed during refining molten pig iron in the converter.

CONSTITUTION: The slag produced in a molten steel-making process which has been formed during blowing pig iron containing at least carbon and phosphorus as a main raw material in a converter is wholly or partially left in the furnace to use it as a desiliconizing agent, and fresh molten pig iron is poured in the converter and desiliconized preliminarily. This desiliconizing agent is generally used by about 20W50kg per ton of molten pig iron, and other desiliconizing agents may be conjunctly used up to a ratio of about a third of said desiliconizing agent.