Oxygen is included in auxiliary gases, and zinc-plated steel plates (3a, 3b) with no blow holes can be welded. As the auxiliary gases, an oxygen gas bomb (4a) for O2 is provided in addition to an argon gas bomb (4b), argon gas and oxygen gas are mixed in a mixer (6), the mixture is delivered to a working head (7) and injected to a working point (9) via a nozzle (8). Oxygen oxidizes zinc of the zinc-plated steel plates (3a, 3b) to produce zinc oxide and zinc peroxide, which do not evaporate, the spatters are decreased in quantities and laser welding with a fewer blow holes can be performed.