To provide a steelmaking slag which is fully carbonated to the interior thereof and usable for civil engineering applications and marine applications for which the slag is pulverized after the stabilization treatment.

A gas containing carbon dioxide is blown in the interior of a molten-state steelmaking slag accommodated in a closed vessel so that the blow-in amount of carbon dioxide becomes ≥0.07 ton per 1 ton of the steelmaking slag; thereafter, the steelmaking slag is cooled to a temperature of ≤860°C. Thereby, the content of free CaO is made to be ≤1.5%, and the content of CO2 present in a form of a carbonate salt is made to be ≥6.3%.