PURPOSE: To test an arbitrary channel of a trunk line from a line repeater and to continue the operation of other channels, by connecting a line signal insertion and pickup circuit and a single channel digital trunk line test device to a digital trunk line relay device.

CONSTITUTION: Telephone exchangers 1 are linked with a digital trunk line 5 via a digital trunk line relay device 3, and the relay device 3 is connected with a digital trunk line signal insertion pickup circuit 8 and a single channel digital trunk line test device 10. This circuit 8 is constituted with an electric switch means, the line 5 between the exchangers 1 is connected, the connection of a desired one channel is disconnected at the test and the test device 10 is connected to the channel. An arbitrary channel of the line 5 can be tested from the relay device 3 without interferring the operation of the other channels.