A light waveguide includes: a light introducing portion for introducing light coming from a light source into a waveguide main body; a light radiating surface for radiating the light introduced through the light introducing portion to the hydrogel side; a light transmitting surface which is disposed opposite to and in parallel to the light radiating surface and through which external light incident from the light radiating surface is transmitted to the exterior; a light separation portion, which provided on a surface in a region ranging from the light introducing portion to the light radiating surface and the light transmitting surface, reflecting only totally reflected components of the light introduced through the light introducing portion; an irregular reflection portion provided on the light transmitting surface to change the reflection angle on the light transmitting surface of the light reflected by the light separation portion; and a mirror portion provided at a position opposite to the light introducing portion.