A portable device transmits wirelessly pilot signals of a plurality of frequencies sequentially at predetermined time intervals. Upon receiving the pilot signals, the in-vehicle device determines whether a slope of a straight line obtained by linearly approximating a time and a reception signal strength of each of the pilot signals is equal to or less than a predetermined slope threshold value. In response to a predetermined allowable condition including the slope being determined to be equal to or less than the slope threshold value being satisfied, a distance between the portable device and the in-vehicle device obtained by a distance measurement process is specified as a high reliability distance. In response to the predetermined allowable condition being not satisfied, the distance obtained by the distance measurement process is prevented from being specified as the high reliability distance or the distance measurement process is prevented from being executed.