An ISDN switching system having facilities for testing an ISDN signal composed of B-channels and D-channels at least. The testing facilities can achieve a B-channel test and D-channel test selectively with simple construction by employing a converting unit in the system. The converting unit enables creation of a connection path between a test equipment and an object to be tested through an ISDN exchange and thus the ISDN signals for the test can be treated as usual ISDN signals. Thereby, the test equipment and the object can freely be connected to each other.