Materials transparent to terahertz waves are very limited, and it is difficult to obtain the required performance by selecting the material. Further, it is also difficult to search for a novel material. Therefore, by letting a known material transparent to terahertz waves have a photonic crystal structure and controlling the structure, an optical waveguide having the required properties is provided.An optical waveguide for propagation of far-infrared radiation in the terahertz region, which optical waveguide is made of a fluorinated amorphous polymer. Particularly preferred is a polymer having a fluorinated aliphatic ring structure in its main chain, obtained by cyclopolymerization of a fluorinated monomer having at least two polymerizable double bonds.