PURPOSE: To reduce the capacity of a capacitor by flowing electric current to a coil to attract a ferromagnetic material thereby controlling the pressure upon the work in the welding stage.

CONSTITUTION: When a switch 16 for starting of welding is pushed, electric current flows through a cabtyre cable 18, a stud 1 and its projection 2, and the arc is generated in the part of the projection 2. At the same time, a magnetic field is created by a coil 11, by which the force trying to pull a plunger 10 which is a ferromagnetic material into the center of the coil 11 is generated. Since this force is in the direction opposite from the force of a spring 9, the plunger can be virtually stopped during the generation of the arc if the spring 9 of a proper force is selected. On stopping of the position of the stud 1, the arc spreads further circumferentally and in accordance with the completion of the discharge by a capacitor to certain extent, the electromagnetic forces diminish and the force pressing the stud 1 to the work 19 increases, thus welding both more firmly.