PURPOSE: To detect a fault of a virtual path surely and quickly at a lower- stream of the virtual path by sending an OAM (for maintenance management) of a VP-AIS (fault information of virtual path) as to all busy virtual paths continuously immediately after occurrence of a link fault.

CONSTITUTION: Upon the receipt of a link fault alarm from an optical signal reception circuit 2 or an out of synchronism alarm from a cell synchronization circuit 3, a link fault detection circuit 4 discriminates it a link fault and sends a link fault alarm to a VP-AIS cell transmission interval control circuit 5. The VP-AIS cell transmission interval control circuit 5 starts a table scanning circuit 7 to scan sequentially each word of a VP table 6 from a head address thereby checking whether or not a virtual path relating to each word is in use. When the virtual path in use is found out through the check, its virtual path identifier VP1 is sent to a VP-AIS cell generating circuit 8, which is commanded to generate an OAM cell representing the VP-AIS.