PURPOSE: To use the band efficiently by sending data to be sent when the data to be sent are in existence in the non-use state where no valid information is sent to a transmission line.

CONSTITUTION: An SS signal being a voice control signal from a telephone set via an exchange is set to a ground level and a start signal is sent. An SS signal detection circuit 103 of a multiplexer 100 detects that the SS signal is set to a ground level and informs it to a control means 106. When the SS signal is set to a ground level, the control means 106 controls a transmission buffer 107 not to send data and commands cell processing of the voice signal to a voice cell processing circuit 104. The circuit 104 processes a time slot of the SS signal and the voice signal into a cell. A multiplexer circuit 108 multiplexes and sends the result. Furthermore, when the telephone set is not busy, the means 106 processes only the control signal of the SS signal and sends the result. In this case, the means 106 controls a buffer 107 to send a data cell.