A network system and a routing method are disclosed herein. By a fiber inserted through the inside thereof, low-coherence light is guided and is made to exit to a living-body tissue side which is an observation target through a lens and a prism forming an exit and entrance portion at the tip portion. In an embodiment the target is comprises a polyethylene foam tube or rod with the exterior cylindrical wall covered by a sheath of material, such as paper, that prevents the foam from bending and thereby increases the rigidity of the target.