PURPOSE: To efficiently find a defective line with high precision even in the case of hurried test of many lines by providing an exchange with a simple test instrument which tests the transmission quality of a subscriber line.

CONSTITUTION: In the system where an exchange 20 and a network terminating equipment 11 are connected by a subscriber line 1, the exchange 20 is provided with a simple test instrument 21 which tests the transmission quality (bit error rate) of the subscriber line. The measurement time and the threshold of the bit error rate can be set to the simple test instrument 21 and the instrument has a function to detect a suspicious line. The measurement time of the simple test instrument 21 is varied as necessary to the test to decide which degree of precision the line is suspicious in. Then, it is preferable that the measurement time is first shortened and is next extended to search defective lines and the search operation is repeated by the required number of times to finally contract the range of suspicious lines.