To provide a welding gun for a robot easily capable of welding without adjusting a travel route of the robot by correcting a position of the welding gun for the robot corresponding to a consumption margin of an electrode.

The welding gun consists of a lock pin 11 fixed to an equalizing lock cylinder 8, which is fixed to a pressurizing cylinder 1, a first piston 13 arranged with an engaging hole to engage to the pin, a second piston 16, which slides the first piston in the axial direction of the cylinder, a ball screw nut 17 turnable in the peripheral direction of the cylinder and a ball screw 9 mounted to a robot arm 7. Further, the welding gun in provided with the equalizing lock cylinder which, by supplying/discharging pressurized fluid to both side chambers of the second piston, a fixed arm 6 together with the equalizing lock cylinder is displaced for a robot arm and the fixed arm 6 together with the equalizing lock cylinder is locked for the robot arm.