PURPOSE: To easily test an ISDN line trunk device without moving a test equipment neither switching the line connection by providing the ISDN line trunk device with a line switching means and operating this means by the indication from the central processing controller of a digital exchange.

CONSTITUTION: Lines from an ISDN network 6 are stored in ISDN line trunk devices 3. Each device 3 has an output terminal for connection to the test equipment independently of connection to the ISDN network 6, and plural ISDN line trunk devices 3 are connected to the test equipment 4 by multiconnection. At the time of testing one trunk device 3, the trunk device number is inputted from a maintenance console 5. A central processing controller 1 indicates line switching to the pertinent trunk device 3 when receiving this trunk device number, and the indicated trunk device 3 switches the line connected to the side of the network 6 to connect it to the test equipment. Thus, the desired trunk device is tested by the test equipment 4.