Disclosed is a technique for quickly detecting a defect of a portion welded by laser beam welding. Specifically disclosed is a welding step in which a welding device is used, the welding device welding an object to be welded with a pulsed laser. In the welding step, a first photodetector receives only infrared rays having a wavelength allowing detection of a keyhole formed in a molten pool from among infrared rays radiating from a welded portion during the welding of the object, and an analyzer determines quality of the welded portion on the basis of intensity of the infrared rays received by the first photodetector.