PURPOSE: To manufacture a wide plate continuously by overlapping side edges of plates to be joined fed from adjacent plural feeding sections, pressing the overlapped part by seam roll to perform plastic deformation the overlapped part in the direction of thickness and pressure welding the members to be joined.

CONSTITUTION: Feeding rolls 1, 2 are so set that adjacent side edges of coil materials 3 and 4 are overlapped by length (l), and coil materials 3, 4 are led between a supporting roll 5 and a seam roll 6. When rolls 1, 2, 5, 6 are rotated while pressing down the seam roll 6 in the direction to reduce the thickness of the plate, coil materials 3, 4 are plastically deformed and pressure welded to be a wide plate 8, and coiled round a roll 7.