PURPOSE: To emit a laser luminous flux in the optimum intensity and to weld without defect or to averagely heat-treat by continuously varying the laser flux in response to the machining position and emitting it.

CONSTITUTION: A gear 12 is rotated by a pulse motor 11 to synchronously move racks 9, 10. Thus, plates 1a, 1b synchronously move in grooves 4a, 4b in separating or crossing direction, the area of passage 14 alters from the maximum value before passing the notch 2a of the plate 1a to 0 of sectional area, thereby throttling the luminous flux of the laser light L. First, the light L is throttled to form a proper intensity at the end to be machined, welded to gradually expand the passage 14, to reduce the passage as approaching the other end through a welding center to machine. Then, no improper working such as holing occurs.