An insert molding method includes: integrally molding a thermoplastic resin with an insert component, by setting the insert component, which is a metal conductor, in a mold; clamping the mold; and injection-filling the thermoplastic resin in a cavity. The insert component is made of copper or a copper alloy. Prior to injection molding, a thin film layer mainly made of a chlorosulfonated polyethylene is formed on a surface of a portion of the insert component to be covered with the thermoplastic resin, and the thin film layer is crosslinked in the mold during injection molding.