PURPOSE: To arbitrarily set a digital test pattern to be sent, and to display a received pattern by adding an arbitrary loopback test pattern input/output function to a testing terminal.

CONSTITUTION: The user line of a user terminal 1 is drawn into an access point 4 for test by an instruction from the testing terminal 8 and the control of a controller 5. Further, it is led to an I interface testing device 7 through a test line drawingin device 6. Then, a loop-back point is set from the device 7 by the control of the controller 5, and the desired digital test pattern from the terminal 8 is inputted, and the digital test pattern is sent from the device 7 under the control of the controller 5. This sent test pattern passes through prescribed route (arrow mark), and is looped back at line terminating equipment 2, and is returned to the device 7. In the device 7, it is compared with the sent test pattern, and the judged result of coincidence or non-coincidence and the received pattern are outputted and displayed on the terminal 8 under the control of the controller 5.