PURPOSE: To make effective use of a channel of a digital line by allocating an optional channel in a digital multiplex line as a test channel and transmitting one bit in the test channel as a test information bit.

CONSTITUTION: In figure, digital multiplex line 3 has 128-channel and a multiplexer 5 accommodates 128-line and assigns a channel number as to each line. A voice of a test personnel is inputted via a microphone 12 and test information is inputted via a test operating device 10 to a per-channel CODEC9. The per-channel CODEC constitute one channel in 8-bit comprising a 7-bit digital signal of the voice of the test presonnel and 1-bit test information and transmits the signal onto a talking channel between a master station test device 7 and a terminal test device 8 set at the start of test. When this channel is received in the device 8, the channel is divided into the 7-bit of voice and the 1-bit of test information at the device 9, the voice is given to 13 and the test information is given to a test circuit 11, and the result is displayed.