To restrain welding failure when making a pipe by welding means.

An image including gradually approaching both side edges 41 of a strip metal plate 40 before being molded in a cylindrical shape by molding means, is photographed. On the photographed image, by image processing, respective positions of both side edges 41 are detected outward in the width direction with a predetermined position between the gradually approaching both side edges 41 of the strip metal plate 40 as a temporary central position y₀ and its temporary central position y₀ as a starting point, and a central position y_ave of a position of these detected both side edges 41 is predicted as a butting expected position. The welding means is positionally adjusted by operating position adjustment means so as to become a welding position when the predicted butting expected position y_ave passes through a position of the welding means.