PURPOSE: To produce a thick steel plate having different sizes in width with good yield by repeating the rolling wherein a rolling material is formed with thin-walled recesses and is rolled after the material is rotated 90°.

CONSTITUTION: After a heated slab 11 is subjected to 2W3 passes of forming by rolling in its longitudinal direction, thin-walled recesses 13 are formed in the prescribed positions in the intermediate part of the rolling material by exertion of strong screwing-down thereupon. The resultant rolling member 12 is rotated 90° and is subjected to broadside rolling. Since the recesses 13 are thin-walled, the broadside spreading is smaller as compared to the thick-walled parts, and a rolling material 14 formed with constricted parts 15 on both sides is obtained. Such material 14 is rotated 90° and is subjected to forming pass rolling. With the parts 15 as a boundary, the rolling reduction is changed and a rolling material 16 having wall thicknesses changing in the rolling direction is formed in said rolling. In succession the material 16 is rotated 90° and is subjected to broadside rolling and the resultant material 17 is further rotated 90° and is subjected to finish rolling down to a finish thickness, whereby a dissimilar width steel plate 18 having sharp stepped parts 19 is formed.