In a sheet-shaped light guide (95), which includes particles for scattering light in a sheet-shaped optical medium and propagates signal light incident from one end face of the sheet-shaped light guide to the other end face side while scattering the signal light by the particles, a high-refractive-index portion 95a and a low-refractive-index portion 95b are formed by the use of a plurality of optical mediums whose refractive indexes are different from each other. Then, an interface between these high-refractive-index portion 95a and low-refractive index portion 95b is curved, and a lens effect for the signal light is imparted.