To provide a monitoring method/device to solve the problem that in the case of welding a butting part of sheet metal work for example by YAG laser welding robot or the like, even the gap or the melt down is bigger than the standard size, the welding is continued as it is without any regard to possible defective welding following the prescribed working program.

In the nozzle 4 of laser beam working head 2, an optical sensor 11 such as photodiode or infrared sensor or the like is arranged to detect the reflected light from the surface of the work W. Then, the detected light is introduced to the control unit, generation of defective welding is foreseen from the prescribed threshold, and suspension of working by the apparatus, or reoperation of welding or so by enlargement of spot diameter of converted beam and supply of filler metal and the like is commanded.