The invention is intended to make jigs for the side members and for the roof lightweight and compact and to facilitate the analysis of problems concerning the assemblability of the side members and the roof. In the first step, clamp jigs 11-1 to 11-4 are used to position a pair of right and left side members 4 and weld them to an underbody 3 positioned at a predetermined position in a vehicle body assembly line 2. In the subsequent second step, the clamp jigs 11-1 to 11-4 are opened and then a roof 31 is placed between the upper weld edges of the right and left side members 4, while using beam-like jigs 35, 36 for positioning and welding.
