A method and apparatus for laser welding, whereby high welding strength, a low degree of variation in strength, and lack of occurrence of burrs can be achieved when two members formed of thermoplastic resin material are welded, are provided. In a laser welding apparatus 1, contact surfaces 4 of a first member 2 formed of transmissive thermoplastic resin that transmits a laser beam and a second member 3 formed of absorptive thermoplastic resin that absorbs a laser beam are joined by melting via laser beam R. The apparatus is equipped with a casing 5 that accommodates the first member and the second member, a vacuum pump 6 that reduces the pressure inside the casing, and a laser beam generator 10, whereby at least one of the contact surfaces of the first member and the second member becomes melted. In the apparatus, the first member and the second member are made to come into contact with each other under a reduced pressure atmosphere, and contact surfaces of the two members are irradiated with laser beam R from the side of the first member that transmits laser beam.