A water-repellent film is formed on a first joint surface of a first member that contains any one of a macromolecular material, a metallic material, and an inorganic material. The water-repellent film is subjected to an activation treatment, and is thereby converted into a film having a high hydrophilicity and a nylon 6 contact angle of less than 25°. A second joint surface of a second member that contains at least a thermoplastic resin, which differs from the macromolecular material, is placed on the first joint surface having the film, whereupon the second member is heated and melted. Then, the molten second member is cooled and solidified in order to bond the first joint surface and the second joint surface.