PURPOSE: To uniformize a rib formation by providing a pair of the disks rotating by clamping a sheet stock from both faces and arranging as well touchably and separatably a pair of plates equipped with a projection or roller at either outer part.

CONSTITUTION: An upper disk 28 and lower disk 34 are provided on a rolled-in rib forming device 2 and an upper plate 29 and lower plate 38 are respectively arranged at the outer part in the radial direction as well. Projections 29A, 38A are formed on the opposed face of the upper and lower plates 29, 38 and a sheet stock 23 is rotated by interposing it by the upper and lower disks 28, 34. In this case, the sheet stock 23 is brought into contact with the projections 29A, 38A and pushed between the projections, hence it is guided by the row of projections and caught inward. The titled rib at an uniform pitch is thus formed on the periphery of the flat part of a spiral cover 3. Namely said rib formation is uniformized.