Abstract: A production line for assembling a pair of left and right car components, capable of assembling a pair of high quality car components by on line at high operating efficiency and productivity and capable of controlling jigs and molds with high dimensional accuracy. The production line of this invention related to a production line for assembling a pair of left and right front doors for supply to an assembly line of the white body of a car, is constructed by arranging various processes on one line, which comprises a work tack welding process A comprising a work input device 3 for alternately inputting the left right front door panels LDP and RDP, an incremental welding process B for welding either one of the works while transferring the other, a marriage process C, a door skin input and sealing process D, a hemming process E, a hemming delivery process F, and a front door delivery portion.