PURPOSE: To finish two slit plates at a same time by installing a roller for restraining two slit plates from the plate width direction and a roller for pressing the edges on the slitter surface sides of slit plates from the above.

CONSTITUTION: A milling device 22 for working the plate width edge surface of a thick plate 20 to a prescribed shape is installed in free contact with the thick plate 20 oppositely in the direction of the plate width onto the downstream side of a slitter 16. A pair of restraining rollers 24 in the width direction which restrains two slit plates 20A and 20B in the plate width direction so that two plates are attached on the slitter surface are installed so as to be attached and separated from the thick plate 20. Further, a pressing roller 26 which press the edges of the slitter surface sides of two slit plates 20A and 20B from the above is installed in free elevation and free transfer in the plate width direction. Further, a plate feeding device 28 which supplies two slit plates 20A and 20B one by one in succession is installed.