A photo detecting device comprises a photo-electron generating body, which absorbs light and generates photo-electrons, and a transistor, across which an electric current flows in accordance with a quantity of the photo-electrons having been generated by the photo-electron generating body. An electrical insulator is formed on a surface of the photo-electron generating body. The transistor is provided with a source electrode and a drain electrode, which are formed on the electrical insulator having been formed on the surface of the photo-electron generating body. The source electrode and the drain electrode of the transistor are connected with each other via a channel section containing a nano-tube having electrically conductive or semi-conductive characteristics.