An apparatus for welding austenitic stainless steels or magnetic materials while feeding a filler metal or workpieces being dissimilar in a quality of material, comprising: a laser beam irradiation section configured to emit a laser beam; and a spectroscopic apparatus configured to receive plasma emission from a molten pool formed by laser beam irradiation, and to acquire optical spectral information, characterized by a measuring section configured to measure a magnitude of the molten pool; and an estimation section configured to estimate material components of a weld zone on the basis of the optical spectral information, to estimate a property of the weld zone on the basis of the material components and to estimate a failure/no-failure of the weld zone on the basis of the estimated property and a measurement result of the measuring section, wherein when austenitic stainless steels to be welded are welded together while feeding the filler metal, the estimation section is configured to estimate a ferrite content on the basis of the optical spectral information and to estimate a hot cracking property of the weld zone on the basis of the ferrite content, when workpieces to be welded are dissimilar in a quality of a material are welded together, the estimation section is configured to estimate a dilution rate at the molten pool on the basis of the optical spectral information and to estimate a strength property of the weld zone on the basis of the dilution rate, or when magnetic materials to be welded are welded together while feeding the filler metal that is a non-magnetic material, the estimation section is configured to estimate a content of the non-magnetic material on the basis of the optical spectral information and to estimate a saturation magnetization of the weld zone on the basis of the content of the non-magnetic material. a laser beam irradiation section configured to emit a laser beam; and a spectroscopic apparatus configured to receive plasma emission from a molten pool formed by laser beam irradiation, and to acquire optical spectral information, characterized by a measuring section configured to measure a magnitude of the molten pool; and an estimation section configured to estimate material components of a weld zone on the basis of the optical spectral information, to estimate a property of the weld zone on the basis of the material components and to estimate a failure/no-failure of the weld zone on the basis of the estimated property and a measurement result of the measuring section, wherein when austenitic stainless steels to be welded are welded together while feeding the filler metal, the estimation section is configured to estimate a ferrite content on the basis of the optical spectral information and to estimate a hot cracking property of the weld zone on the basis of the ferrite content, when workpieces to be welded are dissimilar in a quality of a material are welded together, the estimation section is configured to estimate a dilution rate at the molten pool on the basis of the optical spectral information and to estimate a strength property of the weld zone on the basis of the dilution rate, or when magnetic materials to be welded are welded together while feeding the filler metal that is a non-magnetic material, the estimation section is configured to estimate a content of the non-magnetic material on the basis of the optical spectral information and to estimate a saturation magnetization of the weld zone on the basis of the content of the non-magnetic material.