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.