Abstract: In opposition to a lower electrode (31) fixed to a frame (3), an upper electrode (29) is carried by an upper pressing base (23), which is engaged with a vertical guide rail (25) and joined by a resilient slide mechanism to a piston rod (35L) of a hydraulic cylinder (35) connected to a hydraulic circuit operable for an increased distance (L3) between electrode shanks (15, 7) and fastened at a head end thereof to a lower pressing base (33), which is engaged with the guide rail and joined to a piston rod (37L) of an air cylinder (37) fastened at a head end thereof to the frame and connected to a pneumatic circuit operable for supplying a balancing pressure to have the piston rod (37L) thereof contracted to stand with a total load and for relieving the pressure when a mechanical valve (85) is operated with a pedal arm (49) stepped down for starting a welding, while the pneumatic circuit is changed over by a limit switch (57) to force down the lower pressing base.