PURPOSE: To apply line wire communication with other slave transmission device without giving disturbance to extension communication by starting the line wire communication between a slave transmitter and an external communication network after the synchronization is established by a synchronizing means.

CONSTITUTION: When an incoming request comes from an external line to a slave transmission device 2-k and a signal is inputted to an line wire input terminal 26, a digital phase synchronizing circuit 20 is synchronized with the signal clock to activate a frame synchronizing signal detection circuit 21 and a network side transmission circuit 17. A control circuit 23 inhibits line wire communication in this stage. A phase difference detection circuit 22 detects the phase difference between the frame synchronizing signal from the frame synchronizing signal detection circuit 21 and the frame synchronizing signal at the communication network and gives the result to the circuit 23. The circuit 23 changes the frequency of a clock generating circuit 24 until the phase difference is converged into a prescribed value thereby changing the frame phase of a frame synchronizing signal generating circuit 25. Thus, when the output of the circuit 22 is converged to a prescribed value, the circuit 23 starts the transfer of the information between the transmission device 2-k and a public telephone communication network.