Abstract: A front floor panel (31) and a first small component such as a second cross member (35) to be assembled to a surface thereof, with the first small component at the bottom side are positioned and set into a locator jig (29) at a part setting station (S), a moving stage (27) is moved to a welding station (T), and the front floor panel (31) and first small component are welded. A handling robot (55) reverses the front floor panel 31 front-to-rear, the moving stage (27) is caused to retreat to the part setting station (S), and a second small component such as a front floor reinforcement (41) to be assembled to the rear surface of the front floor panel (31) is positioned in locator jig (29), the moving stage (27) being moved to the welding stage and the reversed front floor panel (31) being positioned in a locator jig (29), after which the front floor panel (31) and second small component are welded together.