PURPOSE: To reduce production cost and to improve flexibility and effective transmission area ratio by packing an optical transmission material having the refractive index higher than the refractive index of a plastic tube into the space formed of said tube and the plugs inserted into both ends.

CONSTITUTION: A plug 2 is inserted and fixed into one end of a tube 1 and an optical transmission material 3 is packed therein from the other end and thereafter another plug 2 is inserted into said end to seal the same. No space is formed between the material 3 and the plug 2. If the material 3 is a high polymer material having no fluidity, said material is difficult to pack and it is preferable to use a method of polymerizing a liquid monomer to a high polymer by polymerizing the same after filling the same into the tube. A material having the refractive index lower than the refractive index of the optical transmission material is selected as the flexible plastic material. The plug 2 is required to be transparent with respect to the transmitted light and the material 3 is selected according to the light desired to be transmitted and the refractive index of the tube to be used. The cost of production is thus reduced and flexibility and effective transmission area ratio are improved.