A welding device for a stretch bender is disclosed. The welding device for the stretch bender may include: an operating unit adapted to move forward toward or rearward from a bending die for shaping a shaped beam and mounted in front of the bending die; and a welding unit mounted at the operating unit and welding flange portions of the shaped beam that is completely formed by the bending die so as to prevent the flange portions from being overlapped with each other when extracting the shaped beam, the welding unit being movable forward or rearward against the operating unit.