The invention relates to an apparatus for laying an optical fiber unit with a pressure fluid into a previously laid duct line. Specifically, the apparatus performs this operation by a method for causing pressure fluid to flow backward into the guide tube on the feed side of the optical fiber unit so as to generate a buoyant force, so that the load due to the frictional force in the section between the feed side and the driving means can be much reduced. Accordingly, the method according to the present invention is particularly useful for pressure-sending an optical fiber unit into a duct line having insertion resistance, for example, a vertical duct line, a duct line having fine curves, or the like.