Abstract: In heating a material radiant at high temperatures, such as a steel product, in a heating furnace, the said material is charged into the heating furnace to heat. A discrimination mark is previously drawn on the surface of material to be discriminated on a portion readily watched from outside the furnace. A heat resisting paint sensitive to near ultraviolet rays is used to make the mark. The said material radiant-heated in the heating furnace is discriminated from outside the furnace as being radiant by applying near ultraviolet rays of a high pressure mercury lamp thereto from outside the furnace to provide a sharp contrast between the said material surface and the said discrimination mark for the discrimination of the said mark from outside the furnace.