In a system where mobile stations use one frequency to transmit/receive and share position information data, synchronization has been made in a shared time resulting from GPS and the mobile stations transmit the data at mutually different timings that are predetermined, whereby overlapping of the transmissions is avoided. However, the transmission period of a station cannot be changed even if that station wants to change its transmission period. Transmission period determining units are disposed which determine transmission periods on the basis of the moving speeds of mobile stations. The mobile stations are time-synchronized by GPS. After the time synchronization, fundamental transmission periods comprising plural time slots are disposed. Transmission is done just in one time slot within one fundamental transmission period normally, and the number of transmissions is increased to two times or three times within one fundamental transmission period when the period is to be shortened.