PURPOSE: To test all lines in a way of preventive maintenance by sending a result of detection of a channel noise detection circuit to a central control circuit so as to implement the control of a test drawing circuit, the start of the channel noise detection circuit and the output control of the result of detec tion to a typewriter automatically.

CONSTITUTION: An outgoing line of a test connection channel of a subscriber circuit (LC) 1 connects to test connection channel outgoing lines in multiple of other LC 1 and connects to an incoming line of test drawing circuit (TL) 2. A channel noise detection circuit (NDT) 4 is accommodated to the outgoing line of the TL 2 and the information as to the start of the NDT 4 and the result of noise detection is sent/received to/from a central control circuit 6 via a control line 5. Then the result of detection of the channel noise detection circuit 4 is sent to the central control circuit 6 and the central control circuit 6 controls automatically the test drawing circuit 2, the start of the channel noise detection circuit 4 and the output control to a typewriter of the result of detection. Thus, the test as to the preventive maintenance of channel noise in a telephone exchange is conducted effectively.