PURPOSE: To facilitate the operation by discriminating whether a voice signal transmitted to a telephone service channel is a data signal or not and prohibiting the mutual connection of voice signals between private lines and public lines based on the result.

CONSTITUTION: A time division multiplexing device 101 controls the switching route of a time switch 108 by an address control memory 110 and performs a repeating transmission of the signal transmitted to a private line 102 to a private line 103 and a public line 104. A voice signal channel is extracted by monitoring a telephone service channel by a voice data discrimination circuit 109 in a signal for which a repeating transmission is performed between the lines 102 and 104. The memory 110 is rewritten so that a repeating transmission may not be performed for the channel and the mutual connection of the voice signal between the private line and the public line is prohibited. Thus, the voice signal and data signal can be set to an arbitrary channel within the channel of the telephone service.