PURPOSE: To perform transmission of service signals (signals for maintenance etc.) between any optional stations without exerting influences on transmission of main signals by adding time slot of the first and second channels comon in each station.

CONSTITUTION: In case where a master station transmits time division multiplex signals and each slave station regenerates a timing signal and sends out signals to the master station only during a time SFA, SFB,... SFn allotted to each slave station, CH1 and CH2 are provided as channels common to slave stations. A slave station (b) (a slave station that uses the common channel first) transmit to the master station through CH2, and the master station transmits the signal received through CH2 to each slave station through CH1. A slave station (c) replying to the signal of CH1 transmits to the master station through CH1. The master station transmits the signal received through CH1 to each station through CH2. The slave station (b) can communicate with optional slave station by monitoring CH2. CH2 is used for communication between the master station and slave station (b) and CH1 is used between the master station and slave station (c).