PURPOSE: To allow central processor to confirm the one-dimension position of a vehicle by simply providing a radio oscillator without requiring any position detecting mechanism on the vehicle side or any method for modulating and transmitting positional information.

CONSTITUTION: A receiving processor 2 modulates a radio wave received from a vehicle 6 by a clock pulse signal received from a CPU (central processor) 1 and sends the modulated wave back on a transmission line 4. The CPU 1 measures the time elapsed after transmission of the clock pulse before reception of the modulated clock pulse signal thus determining the time required for the clock pulse signal to reciprocate between the CPU 1 and the receiving processor 2. A receiving processor 2 which received radio wave from the vehicle 6 can be specified among a plurality of receiving processors 2 based on the time thus determined.