PURPOSE: To relieve the load of a common controller by sampling a scanning signal of each line from a time division multiplex scanning signal and inputting only changed information of the result of sampling to the common controller.

CONSTITUTION: A scanning signal of plural subscriber terminal devices 10∼1m is multiplexed respectively by time division multiplex circuits A∼M of subscriber terminal devices 20∼2m via supervising points a∼m and inputted to a scanning signal change detector 4 from a scanning signal route 3 as a time division multiplex scanning signal. The scanning signal change detector 4 controls the time division multiplex scanning signal of a period T from the common controller 6 via sampling control route 7. The detection of change is conducted by the scanning signal change detector 4 based on the result of sampling, and after the change information is stored in a first-in first-out device 5, the information is read by the common controller 6.