A method is provided for producing a joined body, including: an injection molding step of injecting a thermoplastic resin composition into a cavity of a mold in a state where a solid joining agent containing a thermoplastic resin having an epoxy equivalent of 1,600 or more or containing no epoxy group is placed on a wall surface in the cavity to obtain a first base material in which a resin molded body made of the thermoplastic resin composition and the solid joining agent are integrated; and a joining step of melting and then solidifying the solid joining agent in a state where the solid joining agent of the first base material is in contact with a second base material to obtain a joined body between the first base material and the second base material.

A thermoset sealant film includes an epoxy and a hardener. The hardener is integrated with the epoxy and remains physically distinct from the epoxy in a formed state of the thermoset sealant film.