A portable machine has a signal receivable area in which, when the portable machine approaches one of a plurality of LF transmitters provided to a vehicle at a first distance, a response request signal only from the LF transmitter is receivable by the portable machine and when the portable machine approaches one of the plurality of LF transmitters at a second distance shorter than the first distance, response request signals from the LF transmitter and any of the remaining LF transmitters are receivable by the portable machine. If the portable machine receives the response request signal from only one of the LF transmitters within a predetermined time period and the response request signal has an RSSI value not less than a threshold, control (door locking/unlocking, engine start, and the like) to the vehicle is inhibited.