PURPOSE: To enable a gun change with the disuse of a cable treatment and to simplify the gun changing work by making the plural gun arms supporting a stationary electrode corresponding to the shape of a welding place attachable and detachable for the gun main body by the clamp member provided to the gun main body.

CONSTITUTION: The gun main body 2 is fitted freely advanceably and retreatably and rotatably to the tip of a robot unit 1 and a gun pressure cylinder 3 is provided at the front end part thereof. On the other band, it is made so that the engaging end part 9a (9b) or a gun arm 6a (6b) can fit to the engaging hole 8 formed on the lower face thereof. On the other hand, a clamp member 7 is driven by the cylinder 14 for clamping and the gun arm 6a is fitted by bringing it into contact with the peripheral edge of the engaging hole 8 by an engaging claw 13 with the operation of the clamp member 7 in the state shown by the continuous line from the state of the chain line. The gun arm 6a can thus be changed with the gun arm 6b according to the shape of the welding place and the cable treatment is disused.