To easily monitor a line alarm by specifying a slave interface part by an alarm monitoring device at the time of detecting an alarm and then allowing the alarm monitoring device to mask a spread alarm detected by the slave interace part.

The alarm monitoring device 5 executes alarm collection based on master-slave communication. When a master interface part 2 detects a recieve(REC) alarm such as line disconnection, the device 5 specifies a slave interface 3 line-connected to the master interface part 2 through a time switch 4. Then the device 5 collects the alarm state of the specified slave interface part 3, and at the time of detecting a spread alarm such as an alarm indication signal(AIS) based on the REC alarm, masks the AIS. Consequently unnecessary spread alarms in a cross-connection device can be shrunk.