The floor of a motor vehicle which is assembled in a floor assembly station is transported to a subsequent station using at least two welding robots on each lateral side of the floor assembly station. These welding robots are constituted into dual-purpose welding robots for performing both welding work and transporting work. After the assembling of the floor has been finished, the floor is supported at four lateral and longitudinal jack-up points through the workpiece receiving member by the cooperation of these dual-purpose welding robots, and is transported from the floor assembly station to the subsequent station.