PURPOSE: To decrease segregation in the central part of a steel ingot by attaching the heat insulating materials for the inside surface of a casting mold only near the head part on the long side and up to the position shown by the specific equation on the short side.

CONSTITUTION: A heat insulating material 6A is attached only near the head part of a long side 4A of a casting mold 4 having approximately 2 ratio of the length between the long side and the short side. A heat insulating material 6B is attached to the short side 4B up to such position where the relation between the height (h) of the material 4B at the base of the casting mold and the average thickness B of a steel ingot satisfies the equation 0≤h≤B. The cooling rate from the short side is limited by attaching the materials 6A and 6B in the above-mentioned way and therefore segregation is decreased and the discarding rate in the head part is decreased. The yield of the steel plate and the quality of the product thick steel plate are improved by the above-mentioned method.