p so as to control a gap between the joining members p when the laser welding of the joining members p the roller unit 40 is mounted at the mounting frame 5 ahead of the laser optic head 10 and is movable upwardly or downwardly to the mounting frame 5 the roller unit 40 includes a mounting bracket 41 an operating cylinder 43 and a roller 49 the mounting bracket 40 is mounted at the mounting frame 5 ahead of the laser optic head 10 herein the mounting bracket 41 can be slanted with reference to the mounting frame 5 by a predetermined angle in the present exemplary embodiment the operating cylinder 43 includes an operating rod r and is mounted at a side portion of the mounting bracket 41 the operating rod r is mounted at the operating cylinder 43 and is movable upwardly or downwardly with reference to the operating cylinder 43 in addition the roller 49 is connected to a front end of the operating rod r of the operating cylinder 43 and is movably mounted at the operating cylinder 43 through guide means 42 therefore the roller 49 is slidably contacted to an upper surface of the joining members p depending on movement of the operating rod r so as.