PURPOSE:To improve the productivity of a rolling line, by replacing a dull roll declined in its rolling function with a new dull roll whose surface is previously dulled by an apparatus installed separately from the rolling line. CONSTITUTION:When the necessity of replacement of a dull roll 8 of an arbitrary rolling mill 7 working in a continuous rolling line is confirmed, the rolling line is stopped leaving a strip S set in the rolling line; then, the dull roll 8 to be replaced is pulled out of the rolling mill 7 by using a replacing apparatus. On the other hand, a new dull roll is prepared by applying a dulling work with a dulling apparatus to a new dull roll blank so as to round apexes of dulling projection and to regulate the surface roughness to a prescribed value; this new dull roll is inserted into the rolling mill 7 previously freed from its old dull roll 8 by using the replacing apparatus, and then, the prescribed setting work and inspection are practiced. Immediately following to this inspection, the rolling line is restarted so that the strip S is immediately placed under the prescribed rolling condition.