PURPOSE: To maintain the strength of a joint part at a required level and to allow the welding and joining of a stud bolt without generating dents by covering the joint part of a medium object and a metallic plate having a projection with a shielding gas and cooling this part with low-temp. fluid.

CONSTITUTION: The current discharged concentrically to the projection formed at the front end of a metallic member spreads in the direction parallel with the joint surface of the metallic plate while passing the medium object and, therefore, the generated heat disperses and the thermal expansion, i.e., the generation of the dents arising from the local heating of the members to be joined are lessened. Further, the joint surfaces of the members to be joined and the surfaces on the opposite sides are cooled by the cooling fluid while the current flows and, therefore, the influence of the heating arising from the joining is further lessened. Since the joint part contg. the medium object is covered with the shielding gas while the current flows, the joint part is cut off from the oxygen in the air and the degradation in the strength of the welded part is averted.