PURPOSE: To eliminate the wasteful time in continuous steel making and continuous casting and to improve productivity by pretreating molten iron so as to contain Si, P and S below target values, subjecting the molten iron to decarburizaton, component control, etc. in accordance with casting speeds and casting the same continuously successively.

CONSTITUTION: Molten iron is pretreated to contain Si, P and S less than target values, and such molten iron 2 is held in a vessel 1. Thereafter the molten iron 2 of the quantity controlled according to the casting speed of a continuous casting device is run into a reacting vessel 3 where an oxidation refining gas is injected to the molten iron from nozzles 6 whereby said iron is decarburized. If necessary, the molten iron is fed into a secondary reacting vessel 9, where an inert gas is injected thereon to control the concn. of carbon finely. This molten iron is fed directly into a tundish 11, where alloy iron is charged thereto from an alloy iron charging installation 12 to control the components of the molten steel; thereafter the molten steel is cast continuously successively. Thus the decarburization time and the casting time are matched and the molten steel is cast efficiently.