A weld portion detector (20) and a laser beam projector (10) disposed a predetermined distance behind the weld portion detector (20) are moved together. The weld portion detector (20) detects a weld line and the laser beam projector (10) projects on the weld line detected by the weld portion detector (20) for laser welding. Pieces of data on weld positions sequentially detected by the weld portion detector (20) are stored sequentially in combination with times when the pieces of data on the weld positions are obtained and moving speeds of the weld portion detector (20) in a memory. A time when the weld portion detector (20) passed a point at which the laser beam projector (10) has just arrived is calculated on the basis of the times and the moving speeds stored in the memory. The laser beam projector (10) projects a laser beam on a weld position detected at the thus calculated time.