Abstract: In a method for improving bonding strength between a first resin article and a second resin article, the method comprises using a compatibilizer for both a first thermoplastic resin and a second thermoplastic resin, wherein the first resin article is formed from a first resin composition comprising a first thermoplastic resin, and the second resin article is formed from a second thermoplastic resin or a second resin composition comprising a second thermoplastic resin, and at least the first resin composition out of the first and the second resin compositions comprises the compatibilizer, wherein the first and the second resin articles are welded to each other by laser irradiation to form a composite article.

Abstract: The present invention provides a joining method using a method of laser welding wherein two resin molded bodies can be joined with high welding strength. Specifically, this is a method that joins two thermoplastic resin molded bodies using a method of laser welding; the first resin molded body is a laser light transmissive molded body comprising a thermoplastic resin and cellulose fibers with an ?-cellulose content of 80% or more; the second resin molded body is a laser light absorbent molded body containing a thermoplastic resin and a colorant; and the first resin molded body and the second resin molded body are welded by irradiating laser light from the first resin molded body side.