A position detection system is formed by network connection of a plurality of interrogators and a server. An RFID and the interrogators communicate wirelessly, whereby a distance from each interrogator to the RFID is searched to search a position of the RFID from the distance. In order to calculate the distance from the interrogator to the RFID, a signal is oscillated with a frequency corresponding to amplitude of a signal received in the RFID from the interrogator. A frequency of a signal oscillated in the RFID is detected in the RFID or by the interrogator, whereby a distance from the interrogator to the RFID is detected.