PURPOSE: To decide quickly a fault of an exchange controller and a central processing unit by providing a test call at each line group unit and activating each test call start with a shift so as to detect the presence of the response.

CONSTITUTION: A test call start device 4 is connected to a line in line groups accommodated in the exchange controller 1, a test call is generated automatically at a prescribed period and if an expected response is not obtained from the controller 1, the device 4 transmits a fault signal to a fault supervisory device 5. The device 5 transmits the fault signal to the central processing unit 3, eliminates the controller 1 corresponding to the fault signal and gives the result to a central processing unit fault discriminating device 6. It is decided that the central processing unit 3 is at fault when the device 6 receives generation of a fault from the device 5 for prescribed number of times within a prescribed time by shifting the time of test call start from the device 4 by T/n each, where (n) is the total number of the devices 4 and T is the test period.