PURPOSE: To enable the master station to detect a failure immediately when it occurs in a subscriber's line or in a communication signal by installing a to-station directed alarm sending means in an interface means and a to-station directed alarm receiving means in the master station.

CONSTITUTION: An interface means 9 at a subscriber side is connected to the master station 1 at an alarm communication operating company and to subscriber's devices via a line network as a communication medium by asynchronous three-dimensional group signal between the master station 1 and subscriber's devices. Further, a to-station directed alarm sending means 4 detects the occurrence of an abnormality of the line network and the occurrence of an abnormality of asynchronous three-dimensional group signal, and inserts a result of the detection in a particular bit of the asynchronous three-dimensional group signal to be newly constituted to output. Furthermore, a to-station directed alarm receiving means 3 at the master station 1 takes frame synchronization of an asynchronous three-dimensional group signal to be output from the to-station directed alarm sending means 4 to detect the state of the particular bit. With this, when a failure occurs in a line at the subscriber side, the master station at the communication operating company can detect the failure immediately.