To provide a method for treating molten steel for high carbon steel and its cast slab with which the quality of a steel material worked to this cast slab is improved by producing effective oxide and sulfide as an inoculant nucleus in the molten steel and fining the solidified structure in the solidified cast slab to prevent the generation of internal defect, etc.

In the treating method of the molten steel for high carbon steel having ≥0.5 wt.% carbon, after producing Zr oxide by adding Zr into the molten steel 11, Mg sulfide is produced by adding Mg into the molten steel 11.