An apparatus for seam welding is disclosed, in which a metal plate or a surface-clad metal plate in a cylindrical form with a strip-like overlap section S constituted by two overlapping edge portions is passed as process material 4 between pair roller electrodes 1 and 2 via upper and lower wire electrodes 3 for seam welding of the overlap section between the roller electrodes and also between the upper and lower wire electrodes 3 under pressure applied to the section, and also which is provided with a welding current source 8 for supplying a welding current between the roller electrodes 1 and 2. The apparatus comprises a detector disposed at a position 6a a line upstream of connecting the axes 1a and 1b of rotation of said roller electrodes for detecting at least the front end 4a said process material 4 and a control circuit 7 connected to the output side of the detector for counting the start and stop timings of supply of welding current to the roller electrodes 1 and 2 and instructing the start and stop of the welding current supply to the welding current source 8 according to the counted start and stop of the welding current supply to the welding current source 8 according to the counted start and stop timings.