Seam welding method for vehicle door sash

A seam welding method for a vehicle door sash, the seam welding method comprising

• performing seam welding by applying electricity between a first roller electrode and a second roller electrode in a state where a welding target portion is clamped by the first roller electrode and the second roller electrode rotating in conjunction with the first roller electrode, the welding target portion including a plurality of overlapped metal sheets out of regions of the vehicle door sash, the first roller electrode including a first row of a plurality of first projecting portions that are arranged at intervals in a circumferential direction around a first rotational center and a second row of a plurality of second projecting portions that are in locations shifted toward one side in an axial direction of the first rotational center from the first row and arranged at intervals in the circumferential direction around the first rotational center,
• wherein the second roller electrode includes a first opposing row including a plurality of first opposing projecting portions that are arranged at intervals in a circumferential direction around a second rotational center parallel with the first rotational center and face the respective first projecting portions when rotated in conjunction with the first roller electrode, and a second opposing row including a plurality of second opposing projecting portions that are in locations shifted toward one side in an axial direction of the second rotational center from the first opposing row, are arranged at intervals in the circumferential direction around the second rotational center, and face the respective second projecting portions when rotated in conjunction with the first roller electrode.