Abstract: In an assembly line of a vehicle body W, vehicle body parts differentiated in accordance with the vehicle body type are clamped on respective carriages travelling on an assembly line 1 for sequentially executing temporary welding and power-increased welding for assembling the vehicle body W. An exclusive carriage 3A (3B), clamping parts of the vehicle body W in position for temporary welding, causes a reciprocative movement between a part-supplying station S1 and a temporary welding station S2. A common carriage 10A carrying the vehicle body W causes a reciprocative movement between the temporary welding station S2 and a power-increased welding station S3. Welding robots 29, 29 of the temporary welding station S2 perform the temporary welding for connecting the parts clamped in position on the exclusive carriage 3A (3B). Thus temporarily welded body assembly is lifted and transferred onto a side rotary jig 24 and supported in position for power-increased welding.

Abstract: In a first station, a first part is set to a work at a predetermined position thereof, and the work is then conveyed to a second station by a first jig truck, and the first jig truck which has finished such a conveyance, is then returned back to the first station from the second station. In the second station, the first part is spot-welded to the work, and the work thus spot-welded is conveyed to a third station by a second jig truck. In the third station, a second part is set to a predetermined position of the work with which the first part has been assembled, and the work is then conveyed to a fourth station by the second jig truck. In the fourth station, the second part is spot-welded to the work, and the second jig truck which has conveyed the work to the fourth station, is returned back to the second station from the fourth station through the third station.