Abstract: In a mold having a pair of narrow side walls, a method for reducing the mold width during continuous casting of molten metal includes, as a first step, inwardly moving the upper end portion of at least one of the narrow side walls while maintaining the lower end of that side wall fixed. Thereafter, as a second step, the whole narrow side wall is uniformly moved so that the lower and upper end portions move in parallel. Then, as a third step, the lower end portion of the narrow side wall is moved inwardly until the side wall is inclined at a predetermined angle, thus completing the mold width reduction.