A method of watermarking a color image that has at least three components is provided. A network structure is created by connecting each cell to each of its neighboring cells using bi-directional arcs. Between the light emitting or receiving surface of the light emitting or receiving device 37 and a core exposed in one end face of the optical fiber 15 abutting against the positioning surface 33 there is formed an optical path in which a slope 34 of the prismatic protrusion 31A serves as a reflecting surface. The extraction unit extracts, from the explanation information pieces stored in the storage, an explanation information piece corresponding to the combination designated by the designation unit. The plug connector is adapted to be plugged into the receiving space. The extracted digital certificate public information is stored to at least one dynamically-created certificate public information directory.