PURPOSE: To prevent generation of a carburization phenomenon on and under the surface of a steel ingot by using a covering material and riser brick contg. carbon at a specific rate.

CONSTITUTION: A molten steel 2 is charged from the bottom 3 of a casting mold 1 into the mold 1. A riser brick of a top placing type is installed atop the mold 1. Said riser brick has the double-layered construction consisting of a brick layer 4 contg. ≤1.0% carbon and a brick layer 5 which is usually a chammotte brick. A covering agent 6 contg. likewise ≤1.0% carbon is used. The rate of discarding owing to carburization is decreased and the yield is improved.