PURPOSE: To perform accurate welding work by controlling a balancing cylinder of the C-shaped welding gun in which an equalizing unit is interposed between the wrist of a robot and a welding gun main body for any attitude of the welding gun.

CONSTITUTION: The equalizing unit 10 is interposed between the welding gin main body and a robot side operating arm 1. A first solenoid changeover valve 14 is arranged between an air source 17 and a pressurizing cylinder 5 and a second solenoid changeover valve 15 is connected to the discharge side of this changeover valve 14. Both chambers 34 and 35 of the balancing cylinder 11 provided on the equalizing unit 10 are connected to an air-hydro converter 25, respectively and a third solenoid changeover valve 16 and an air-operated valve 36 are arranged in parallel between this converter 25 and the air source 17. The third solenoid changeover valve 16 is controlled by a sensor 4 which operates according to the position of the balancing cylinder 11 and the air- operated valve 36 is operated according to a signal due to increase of the air pressure in the pressurizing cylinder 5.