PURPOSE: To enable the welding profile precisely welded in the full length of the welding groove.

CONSTITUTION: This welding device is for welding the welding profile 3 in the welding groove 15 located at the inner side of the lock. The lock consists of the hollow profile elements 20 and 21 which are fitted together, and especially they are connected together at their layers providing as the fluid-isolation parts in the slit wall. The welding device is equipped with a transfer car 1 which is movable in the longitudinal direction in the inner profile element 20. The transfer car is equipped with the heater 10 and the pressing device 5 for the welding profile 3. The clamp 6 is installed to support the lower flattend side of 3a of the welding profile 3, and located in front of the heater 10 and the pressing device 5 in the welding direction 10. The welding profile 3 is guided to the supporting device 6 from the heater 10 via the pressing block 4 of the pressing device 5.