In order to provide a portable electronic device improved in water tightness by a simple method, the portable electronic device includes: a housing assembled from a plurality of housing pieces via a sealing member and having a built-in electronic circuit; and a flat cable group in which a plurality of flat cables having a plurality of electric signal lines aligned and connected to the electronic circuit are stacked in a thickness direction, and adjacent flat cables are bonded to each other via an adhesive material at least at a point in an overall length, the flat cable group being integrally molded with the sealing member of the housing at the point.