To provide an illumination device of a cooling/refrigerating open show case which does not affect any articles on display due to the radiation heat, Is capable of preventing the load in cooling the show case, easy in maintenance and inexpensive.

An optical transmission cable 1a in which an outer circumference of a flexible and transparent core 2 rich in permeability is covered with a tube 3 having a high reflectance factor and made of Teflon and a protective layer 4, is used. One end of the optical transmission cable 1a is arranged in a light source device, and the other end is an illumination part 7 by peeling the protective layer 4 and the tube 3, and attaching a reflective tape 6 to the opposite side of the illumination direction. Illumination is provided by installing the illumination part 7 on a lower part of a shelf plate 13 of the show case, a recessed part of a ceiling part, etc.