A waterproofing structure in a core wire includes a covered electric wire in which a plurality of core wires are covered with an insulating covering, a core wire bundle expanded portion in which the core wires of an intermediate core wire exposed portion are separated from each other and expanded in diameter by removing a part of the insulating covering, and a mold portion that is molded to surround the core wire bundle expanded portion by cooling and solidifying a thermoplastic adhesive filling between the core wires in the core wire bundle expanded portion by hot melting, together with the insulating coverings at both sides between which the intermediate core wire exposed portion is interposed.