PURPOSE: To obtain with a good yield and easily a thick steel plate provided with a groove, having a high accuracy, by using a width rolling reduction roll with a caliber, and a holding roll.

CONSTITUTION: A width rolling reduction roll 3 having a flat part 3a and a caliber part 3b, and holding rolls 11, 12 for preventing an offset of a rolling steel material 1 are incorporated in a cartridge 13, and this cartridge 13 is placed on a truck 24. A position of the caliber part 3b and a height of the steel material 1 are adjusted by shifting said whole cartridge 13 upward and downward by a cylinder 21, and on the other hand, as for the rolls 11, 12, the related position to the roll 3 can be adjusted independently by shifting them upward and downward. First of all, the upper face and/or the lower face of the steel material 1 are held by the rolls 11, 12, also a path level of the steel material 1 is kept constant, and a chamfering rolling of the side end edge part of the steel material 1 is executed, by which a thick steel plate with a groove is obtained.