PURPOSE: To reduce man-hours for manufacturing an H-steel by running a truck to which a guide roller that copies a flange center on the web of a tacked H- steel is attached and welding fillet parts of the H-steel by plural torches attached to the truck.

CONSTITUTION: The truck 6 is run on the web of the H-steel 1 having flanges 3A, 3B tacked to both ends of the web 2. Guide rollers 7A, 7B that copy the flange center positioned at the center of the web 2 are attached at the center of the truck 6 through a supporting fitting 8 and a web width adjusting mechanism 9. Further, two welding torches 10A, 10B are attached through a welding torch adjusting mechanism 11 in the rear part, facing the direction of running, on the truck 6, and tips of the torches 10A, 10B are directed respectively to fillet parts of the web 2 and flanges 3A, 3B. By this constitution, the truck 6 is run by the guide rollers 7A, 7B, both fillet parts are welded simultaneously. Then, the H-steel is inverted, and opposite fillet parts are welded similarly.