To stably achieve the reduction of the inclusion sizes in a slab by coating the outer periphery of a metal Al grain alone or an aggregate of ≥2 metal Al grains with MgO, adding these grains into molten steel to deoxidize the molten steel.

The deoxidizing material prepared by coating the outer periphery of the metal Al grain alone or the aggregate of the many metal Al grains which are preferably as fine as possible with the MgO is added by injection from a pipe immersed in the molten steel to deoxidize the molten steel. The injection of the deoxidizing material together with gas from the bottom tuyeres of a vessel for molten steel, such as a ladle, or the injection thereof under reduced pressure is also effective. The yield of Mg is as high as ≥70% and is stable and the sizes of the inclusions in the slab are reduced. The slab drastically decreased in the number of the large-sized inclusions is thus obtained. The slab which is extremely little in the coil defect after rolling and the defect at the time of product fabrication may be obtained. Good results may be obtained even with steel kinds in which the toughness of weld heat affected zones is important.