In a communication system using a sheet light guide 10 which is formed in a manner that the sheet light guide contains light-scattering particles in a sheet optical medium, and propagates a signal light incident from one end surface thereof to the other end surface side to which a photodetector is coupled, while scattering the signal light by the particles, at least two of the sheet light guides 10 are coupled to each other with an angle-converting light guide 90 interposed therebetween.