PURPOSE: To discriminate easily and surely a product whole printed label is relatively deviated from its machining position by monitoring a discrimination mark provided to a prescribed position, in working a printed metallic material for manufacturing a vessel cap having a cylindrical skirt, provided to with a projecting part and drooping from its top face.

CONSTITUTION: A printed 6 blank material 2 of metallic thin-sheet is provided with machining, such as blanking, stamping, and curling, to manufacture vessel caps 14, having each a shape shown in the figure, from plural areas 8. In a projecting area 12 in the area 8, four discrimination marks 36 are provided to a part for regulating the annular part 24 of cap 14, by arranging the marks 36 at 90° angular intervals. An inspector of a cap 14, manufactured by applying said preparation, can find out a reject easily and surely by visually discriminating the marks 36 of a part, where extremely remarkable positional-deviation and deformation appear in case of the reject, without the need for directly monitoring a label 34 on the top face 16 of a cap 14 by the eye. Further, the discrimination can be automated by an optical discrimination system instead of the visual discriminations.