A switching unit (1011) receives video data (103) sent from a device (102), and sends an ATM cell synchronized with the clock to switching units (1012-1014) by a signal (including optical data) containing a frequency component serving as a synchronous clock supply source. If a fault occurs midway along a line, the switching unit (1012) which detects the fault sends an AIS cell to an OAM connection (109). The termination switching unit (1014) switches a port f for receiving a synchronous clock to a port g, and sends back an RDI cell. The switching units (1013, 1012) which have received the RDI cell switch the synchronous clock to ports e and c and receive it, thus obtaining an identical clock.