To solve a problem that there is the possibility of the generation of cracks in the vicinity of a joint on the side of a vessel under the influence of the heat because it is generally performed to increase the value of impressed current at welding in order to surely perform welding in seam welding in conventional technique, but heat generated at welding is increased and the amount of heat is also increased when the current value is increased.

In a roller electrode for seam welding, at least one groove part which reaches the inside side face from the outside side face of the roller electrode is formed on the contacting/energizing surface of the roller electrode in this roller electrode for seam welding.