To provide a roadside-vehicle communication system that can assist in driver's safe driving and precisely inform the driver of a lane when a road has a plurality of lanes.

The roadside-vehicle communication system comprises an on-board device 2 for a vehicle C traveling on a road R, an optical beacon 4 which has a projector photodetector 8, having its communication area A set within a predetermined range on the road R, and provides two-way communication, by using optical signals between the on-board device 2 and the projector photodetector 8 of the optical beacon 4. A vehicle sensor 5 is provided which has its sensing area K at a predetermined position in the communication area A, and the optical beacon 4 is provided with a communication control unit 7, which decides whether a downlink is switch, based on whether uplink information is received or not and whether a sense signal s1 is received from the vehicle sensor 5.