<p>An optical fiber unit to be installed by the drag force of a pressure fluid flowing through a pipe, which optical fiber unit comprises at least one optical fiber and more than one interstitial cord which are bundled and surrounded by an inner sheath and an outer sheath to provide a unitary assembly, which inner sheath is formed of a resin that has a high Young's modulus and that will exhibits small residual strain during the application of sheathing and which outer sheath is made of a foamed polyethylene.</p>
