PURPOSE: To attain easily the test of plural telex subscriber circuits, by switching a line switching device after the test of the telex subscriber circuit is finished in a time division line exchange so as to form sequentially a loop circuit to other telex subscriber circuits.

CONSTITUTION: A test data is transmitted from a personal computer 200 and outputted to the path of a signal line 201, an I/O controller 210, and a signal line 211. The test data returns in the path of an A/D, D/A converter 220, a signal line 221, a line switching device 230, a test terminal 241, a subscriber circuit 240 to be tested, a signal line 251, a subscriber circuit controller 250, a signal line 213, an I/O controller 210, and a signal line 201, the personal computer 200 checks if the receiving data has a prescribed voltage for checking the normality. When the bidirectional test of one subscriber circuit to be tested in this way, the computer 200 transmits an instruction for the designation of the next subscriber circuit to the line switching device via the signal line 201, the I/O controller 210 and a signal 212.