PURPOSE: To make it possible to effectively eliminate air spaces among a plural number of flat masks caused by the inherent warp of the masks so that sliding among the masks and uneven dilation are completely suppressed by closely contacting the masks with each other over the entire surface of them with magnetic force.

CONSTITUTION: Two flat masks are completely contacted with each other to suppress the inherent warp of the masks with magnetic forth, so that there are no air spaces between a platen 2 and a mask 5 and between a mask 4 and the mask 5. In such a state, the two flat masks are mechanically fixed by subjecting mask peripheral parts to seam weld or spot weld, or further by burrying the aperture parts of the masks with resin, and thereafter, a magnetic force generating device is freed, and the two flat masks are subjected to press molding in the same way as that of an ordinary shadow mask for color picture tube, allowing a desired curvature to be given. Two masks constructed with this method scarcely yields the warp of a mask aperture and the position deviation of each aperture corresponding to each of two masks.