An in-vehicle device includes a plurality of transmitting antennas, an in-vehicle device transmitter configured to transmit measurement signals via the plurality of respective transmitting antennas, an in-vehicle device receiver configured to receive, from a portable device, a measurement result signal that includes measurement data of received signal strengths of the respective measurement signals that have been transmitted from the plurality of respective transmitting antennas; and an in-vehicle device controller configured to calculate first distances from the portable device to a plurality of preset areas, based on differences between the received signal strengths of the respective measurement signals included in the measurement result signal, and to identify an area to which the portable device belongs from the plurality of preset areas based on the first distances.