A signal processing unit of a radio communication section of a vehicle-mounted device mounted on a vehicle broadcasts first sets of data, each set of data including a vehicle speed and a traveling direction of the vehicle at a predetermined period when the vehicle changes the vehicle speed or the traveling direction. The signal processing unit broadcasts second sets of data including the current vehicle position, the vehicle speed, and the traveling direction of the vehicle at predetermined timing. When receiving the first data and the second data from a peripheral vehicle, the signal processing unit calculates a position of the peripheral vehicle in accordance with the first and second sets of data.