To provide an on-vehicle device which enables position information communication, even within a given communication zone, and precisely monitors the positions of other vehicles in real time, and to provide an other vehicle position computing method.

The signal processor 15 of the radio communication part 10 of the on-vehicle device broadcasts first data which include the vehicle speed and proceeding direction of its own vehicle with a prescribed cycle, when the speed or proceeding direction of its own vehicle changes, and also broadcasts second data which include the position, the vehicle speed and the moving direction of its own vehicle in specific timing. The above signal processing part computes the positions of other vehicles, based on the first and second data, when it receives the first and second data from other vehicles.