A roll forming system which roll-forms a shaped beam by a roll forming unit including a plurality of roll formers is disclosed. A laser welding device for the roll forming system may include: front and rear guide means disposed apart from each other at the rear of the roll forming unit, and guiding movement of the shaped beam; laser welding means mounted above and between the front and rear guide means, reciprocatedly rotating a mirror by controlling a mirror motor so as to irradiate a laser beam to a welding position of the shaped beam, and thereby forming a welding bead having a predetermined welding pattern; forming speed detecting means detecting a forming speed of the shaped beam and transmitting a signal corresponding thereto to a controller; a camera disposed at the rear of the laser welding means, photographing a welding portion of the shaped beam, and transmitting an image signal corresponding thereto to the controller.