PURPOSE: To improve the cross rolling of a thick plate only by the longitudinal rolling of a slab and the quality of the thick plate product by providing stoppers to upper and lower splined shafts controllably of their moving in and out.

CONSTITUTION: The stoppers 10 on the splined shaft 1, 2 sides fitted in the fitting holes 17 of sleeve rolls 13, 14, 15, 16 are kept retreated in the shafts 1, 2 by the pressure oil supplied thereto. The central part of a material S to be rolled is rolled by a pair of upper and lower rolling rolls 1a, 2a in the central part. A pair of upper and lower sleeve rolls 13, 14 on a left side are moved in a right direction and a pair of upper and lower sleeve rolls 15, 16 on a right side are moved in a left direction. The gaps between the rolls 13, 14 and the rolls 15, 16 are set at the values smaller than the widths of the rolls 1a, 2a. The pressure oil is removed and the stoppers 10 are fitted into the holes 17 of the rolls 13W16 by springs 12. The material S is rolled by the rolls 13W16.