PURPOSE: To enable molten steel level to be detected stably for a long time by burying a temperature sensitive magnetic body in the wall of casting mold.

CONSTITUTION: A hole is opened from the corner portion of a casting mold 1 and a temperature sensitive magnetic body 2 is inserted, after which a high-temperature- resistant and water-resistant adhesive agent 7 is filled. This reduces the contact area of the adhesive agent portion with cooling water 4, thereby preventing deterioration of the adhesive agent 7 by the cooling water and preventing the entry of the cooling water in the portion of the heat sensitive magnetic body 2. And the exciting frequency of the detecting coil 3b provided on the surface of jacket 5 opposing to the heat sensitive magnetic body 2 is reduced, whereby the change in magnetic permeability may be measured and the level of molten steel 6 may be detected even if the sensitivity is lowerd owing to the embedding of the heat sensitive magnetic material 2 in the casting mold 1.