Abstract: A method for assembling a motor vehicle body by fixing relative positional relationship among a plurality of motor vehicle body constituent parts by means of positioning members provided in a welding jig which is disposed on each side of an assembly station and, in this condition, welding together the plurality of motor vehicle body constituent parts at a plurality of tack-welding locations. First welding guns of indirect feeding type are mounted on the welding jig to weld a part of the tack-welding portions. A welding robot having mounted thereon a second welding gun is disposed outside the welding jig. The above-described part of the tack-welding portions are welded by supplying electric power from the second welding gun to the first welding guns by pinching, by the second welding gun, feeder terminals of the first welding guns in a condition in which the above-described part of the tack-welding portions are being pinched by the first welding guns.