An optical-fiber incorporated longer-sized subaqueous unit comprises optical fiber units arranged inside or outside a plastic sheath provided on the periphery of the core body of the longer-sized subaqueous unit such as power cable core or water supply tube. Each optical fiber unit includes an optical fiber accommodated in a metal pipe having on its periphery a coating layer made of plastic material with a melting point higher than that of the material of the plastic sheath, and armoring iron wire bundles are further provided outside the plastic sheath on the periphery of the core body, thereby preventing any buckling in the optical fiber units.