PURPOSE: To execute welding in a short time by a simple welding device, by placing a rolling roll in front of a welding roller electrode, and thinning thickness of a metal to be worked, in a mash seam welding device provided with the roller electrode.

CONSTITUTION: When 2 metallic strips 3, 4 clamped by clampers 1, 2 are pressed by a pair of upper and lower roller electrodes 5, and seam welding is executed, a pair of rolling rolls 7 are additionally provided in front of the roller electrodes 5, and the thickness of the strips 3, 4 is reduced by rolling them before they are inserted between the roller electrodes 5 and are welded. Scrap parts 8, 9 generated in case of welding by electric conduction and pressure using the roller electrodes 5 are cut and removed by a cutting device 6 placed immediately behind said electrodes. According to this device, the thickness of the strips 3, 4 is reduced by the rolling roll 7, therefore, a welding speed by the roller electrodes 5 increases, also cutting resistance in case of cutting the scrap parts 8, 9 becomes small, and the whole device can be made small-sized.