To provide a seam welding head having a structure capable of executing the consistent power supply to and preventing any trouble such as seizure in a supporting structure of a power supply shaft with a roller electrode being fixed to one end, and a power supply holder for rotatably supporting the power supply shaft.

In the seam welding head, a power supply holder 1 has a through hole 1A with a power supply shaft 5 being inserted therein, and the power supply shaft 5 has a large diameter part which is rotated while being slidably brought into contact with an inner wall of the through hole 1A, and a plurality of small diameter parts 5A for storing conductive grease. The plurality of small diameter parts 5A provided on the power supply shaft 5 are formed with the equal width, and provided at equal spacing in the direction of a rotary shaft of the power supply shaft 5.