PURPOSE: To provide a welded part formed nearly uniformly all over the body under a proper molten state irrespective of the surface state of a part to be welded by the laser beam when the welded part is formed by irradiating the part to be welded with the laser light beam.

CONSTITUTION: A part 14 to be welded is irradiated with the laser light beam 16 from a laser beam generating part 15 and welded by the laser beam 16, photoelectric transfer is carried out regarding reflected laser beam 18 from the part 14 to be welded and the output in the laser beam generating part 15 is controlled in accordance with the comparison output obtained by comparing an output voltage obtained by the photoelectric conversion with a reference voltage.