PURPOSE: To connect an arbitrary channel semi-fixedly, by connecting a specific channel of digital signals through the use of a channel semi-fixed connecting device.

CONSTITUTION: In connecting a specific channel of a digital interface 14 fixedly, a CPU19 instructs the set of connection to a channel fixedly connecting device 20. Then, the device 20 stores channel information relating to the specific channel to give a connection control instruction to a time division switch 15 and to set connection in the switch 15. When the specific channel set and connected is called, the device 20 picks up only the specific channel with the channel information for the relay of a control signal. As to the specific channel of the interface 14 from other digital transmission interface 13 or a digital encoder 12, the channel information is stored in the device 20 and the similar operation can be done.