PURPOSE: To improve the quality of a weld metal body by providing a projection to the lower part of a rear clamp and arranging projecting bodies to both side parts of the common base and detaining the projection and projecting bodies to prevent the rear clamp from retreating.

CONSTITUTION: The tapered surface 6a a vertical to the projection 6a of the rear clamp 6 is formed and the projecting bodies 1a are arranged to both side parts of the common base 1 and further, supporting devices 13 are provided to the projecting bodies 1a. Metallic strips (s) and s' held by clamps 3 and 6 and an abutment is cut and a lapping allowance of the prescribed length is formed by a tilt of the clamp 3 and an advance of the clamp 6. Hydraulic cylinders 13a are actuated and supporting bodies 13c are allowed to abut on the projection 6a of the clamp 6. When the lapping allowance part is crushed while being welded with electrode rings 11 and 12, the projection 6a of the clamp 6 and the projecting bodies 1a of the common base are detained and the clamp 6 is prevented surely from retreating. Accordingly, the quality of the weld metal body is improved.