PURPOSE: To relieve lots of faulty digital multiplex group transmission channels with a small installation scale by applying band compression to the digital multiplex next group signal to be sent when a 1st transmission means is changed into a 2nd transmission means.

CONSTITUTION: A cross connector 2 releases channel setting between multiplexers/demultiplexers 1 and 3 in the unit of channel setting unit channel number and sets a channel between the multiplexer/demultiplexer 1 and a digital digital converter 15. Thus, plural 64kb/s digital signals are band-compressed by the digital digital converter 15 and plural digital signals subject to band compression are multiplexed by a multiplexer/demultiplexer 16. That is, number of channels included in the digital multiplex group signal is equivalently increased without revising the signal rate of the digital multiplex next group signal being the setting unit by the cross connector 2 and lots of faulty digital multiplex next group transmission channels are relieved.