PURPOSE: To provide the WALK back system for a mobile set in which the mobile set is located and the tracing of the position is quickly and easily implemented.

CONSTITUTION: The system is provided with a mobile body exchange 2 which stores a caller number in setup information from a mobile set 1 and stores information representing a trace back request to a home memory 4 of the mobile set 1 according to the stored caller number when the exchange 2 receives a prescribed PB signal from a called terminal equipment 3 during the transmission of a busy tone to the called terminal equipment 3 after the busy mobile set 1 hooks on. Furthermore, the mobile exchange 2 sends a signal accessing the mobile set 1 and a prescribed control signal to the mobile set 1 when the information representing a WALK back request is stored in the home memory 4 of the mobile set 1 referenced in the case of implementing processing for dialing, call reception or position registration request with respect to the mobile set 1. Moreover, the mobile set 1 is provided with a timer means 6 energized by receiving processing control signal from the mobile body exchange 2 and sends a prescribed carry signal including ID information of the mobile set 1 as the timer means 6 periodically expires.