PURPOSE:To mass-produce tapered plate material products of modified cross section easily with high precision by using a male roll dowel which has a tilt angle to a shaft core and a female roll dowel which corresponds to it when a tapered plate material having flange parts on both sides is formed. CONSTITUTION:The roll forming device consisting of the male die roll dowel 1 whose one end surface slants to the revolving shaft core at the necessary tilt angle and the female die roll dowel 2 which has a groove 2a meshing with the roll dowel 1 is used; the plate material 3 is held and both roll dowels are rotated in the opposite directions to manufacture plate products which have the flange parts 3a on both sides and vary in sectional shape lengthwise easily and successively. In this case, size L is the circumferential length of both roll dowels 1 and 2, and the long-sized plate material 3 is formed successively and then cut to length L or L/2 to obtain numbers of tapered sashes of the same length.
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