A vehicle wiring system includes a plurality of functional units to be mounted in a vehicle, an optical transmission line that is wired between the functional units and configured to transmit an optical signal of the functional units, an optical coupler that is disposed in a partial section of the optical transmission line and constitutes a part of the optical transmission line, and a structural member that constitutes a part of the vehicle and is at least partially transparent. The optical coupler has an optical waveguide that passes through the structural member.