PURPOSE: To prevent a bottom part or an oblique side from generating the strain in a time of producing the article of a roof material, etc., which is curved in the longitudinal direction.

CONSTITUTION: This roll forming device provides the multi-stage roll parts 6 to 21 which form a long size flat, thin sheet 2 composed of a steel so that the sheet 2 is gradually formed in a front face shape having a high height and an almost V-shape or a corrugated shape, and this device provides an arch forming roll part 26 which forms the formed article in an arch shape by incurvating in the longitudinal direction. Further, rippling waves forming roll parts 1, 23 in order to respectively form the rippling waves like projecting and recessing parts at the proper positions of the oblique side and the bottom part of the arch like article to be formed are arranged.