Welding method, welding apparatus, and a method for producing a battery

A welding process using a welding device (1) for welding the welding object by a pulse laser (S1), the infrared rays emitted from the welded parts during welding of the welded, the keyhole formed in the molten pool only infrared with a detectable wavelength received by the first light receiving device (20), the analysis device (40), based on the intensity of the infrared rays received by the first light receiving device (20), the state of the welded parts judges of the good or bad.