Abstract: A guiding method and apparatus are presented for preventing the generation of lateral curving in a rolled slab produced in a roughing mill train, consisting of an edging mill and a horizontal mill in tandem, for example. The apparatus basically includes a first side guide placed upstream at the entrance to the edging mill, a second side guide placed downstream at the exit of the horizontal mill and a third side guide placed between the edging mill and the horizontal mill. The first side guide at the entrance to the edging mill synchronously closes the separation distance of the guide components upon the entry of the rolled slab into the first side guide, leaving a narrow space between the guide and the rolled slab. The third side guide closes the separation spacing based on slab exit timing representing the instant the tail end of the rolled slab passes through the edging mill. The second side guide assumes a narrow separation distance when the head end of the rolled slab enters therebetween.