RADIO COMMUNICATION SYSTEM

PROBLEM TO BE SOLVED: To increase the reachable range of a request signal from an in-vehicle device to a portable unit while suppressing the radio wave intensity of the request signal within a desired range, and to much more accurately specify the position of the portable unit.SOLUTION: An in-vehicle device 10 performs the spectrum diffusion modulation of transmission data by a predetermined diffusion coefficient, and transmits a request signal in an LF frequency band subjected to the spectrum diffusion modulation. Furthermore, the in-vehicle device 10 gradually changes the diffusion coefficient, and makes an LF transmission section 3 successively transmit a plurality of request signals whose reachable ranges are different, and determines the reception of a response signal to be transmitted by a portable unit 20 in response to the request signal, and divides a region where the portable unit 20 is positioned into a plurality of regions for specification on the basis of the determination result of the reception of the response signal.