PURPOSE: To manufacture easily longitudinally tapered metallic roof members with various dimensions by passing a tapered flat plate one side by one side through a forming roll straightly by using a guide and pressing device.

CONSTITUTION: A flat plate (a) having two longitudinally tapered sides 9 and 10 is transferred to a rotating roll 17 of the first manufacturing device 11 to pass the plate in an arrowed longitudinal direction. One side 9 of the plate (a) is passed straightly on a specified line by means of a pressing device comprising a guide 12, a rotatable pressing roll 18, a rod 19, a bracket 20 constituting a support, and a spring 21 etc., and a rise forming part 9' is formed by a forming roll 15 which is coaxial to the rotating roll 17, then a half-formed flat plate (b) is obtained. Nextly, the plate (b) is passed through the second manufacturing device 11' and a rise forming part 10' is formed on the other side 10 by the same means as above mentioned, and a tapered roof member (c) fitted with the rise forming parts 9' and 10' at both sides is produced.