PURPOSE:To make the automatic decision of whether welding states are good or not possible by irradiating detection light to the members to be welded, and detecting melt diameter from the change in the quantity of the reflected detection light. CONSTITUTION:When a laser beam L is outputted by operating a laser oscillator and the 1st member 12 to be welded is irradiated, the member 12 begins to melt upon lapse of a prescribed time after the start of the irradiation. On the other hand, light projectors 14a-14e are operated together with the laser oscillator and the detection beams l1-l5 reflected by the member 12 are detected with photodetectors 15a-15e. More specifically, the detection beam l3 reflected from the central part of the spot of the laser beam L irradiating the member 12 lowers in reflectivity and the quantity of reflected light as the central part melts earliest. The states of the quantity of the reflected light detected with the photodetectors 15a-15e are arithmetically operated in a single processing and displaying device 16, by which the melt diameter of the 1st member 12 is known. From this melt diameter, whether the molten state is good or not is decided.
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