PURPOSE: To roll a slab into a thick finished plate with less plate-width deviation in the longitudinal direction, by preliminarily rolling the slab before its broadside rolling to make its thickness distribution uniform in the longitudinal direction by adjusting a rolling down position, before broadside rolling its end part in the longitudinal direction into a thick part.

CONSTITUTION: In the preliminary rolling X of a slab SL before its broadside rolling; the slab SL is preliminarily rolled by changing and adjusting a rolling- down position in accordance with the changes of the deformation resistance and inlet thickness of the thick plate due to mainly a presence of skid mark, to obtain a pre-rolling finished material SB before broadside rolling, having the uniform thickness distribution in its longitudinal direction. Successively, the material SB is turned by 90°, and is subjected to rolling reduction in its longitudinal direction by a vertical mill in a broadside rolling Y so that the plate thickness of its end part in the longitudinal direction is larger than that of the central part in the same direction, to obtain a finished material SW in the broadside rolling. The material SW is further turned by 90°, to obtain a finished plate CP, having prescribed dimensions and less plate-width deviation in the longitudinal direction, by a finishing rolling Z.