To obtain an inter-road-vehicle communication system that can properly attain hand-over even on the occurrence of random fluctuations in a received level.

An on-vehicle wireless unit receives a communication channel sent from a 1st road side wireless unit and a had-over channel sent from a 2nd road side wireless unit adjacent to and in front of the 1st road side wireless unit, obtains a relative position of itself on the basis of measurement of a relative time difference between timing markers included in the respective channels, and detects a position at which the hand-over is to be executed without being affected by unstable level of a received radio wave.