In a position detection system, a plurality of first communication devices each comprise a transmitter for transmitting a distance-calculation signal for calculating distance to the second communication device; a detector for detecting, by receiving from the second communication device a signal corresponding to the distance-calculation signal, phase difference between the distance-calculation signal when transmitted and the signal corresponding to the distance-calculation signal when received; and a calculator for calculating the distance to the second communication device, based on the phase difference. A second communication device comprises a transmitter for transmitting the signal corresponding to the distance-calculation signal. The position detection system comprises a determining unit for determining whether the multiple distances are below a predetermined distance; and a position detector for detecting the position of a relay device corresponding to the multiple distances when the determining unit has determined that the multiple distances are at or above the predetermined distance.