PURPOSE: To attain automatic switching of an optical transmission panel by providing a memory circuit storing an alarm sent during active state to an active system and a standby system respectively.

CONSTITUTION: The active and standby systems are provided respectively with inhibition circuits 5, 6 not fetching an optical output interruption alarm at the cold standby side and memory circuits 7, 8 storing the optical output interruption alarm sent during the active state. Then the optical transmission panels 1, 2 take the cold standby and only one panel is set by a switching control circuit 4 and an output is obtained by an optical multiplexer 3. The redundancy switching of the optical panel is implemented by using the optical output interruption alarm sent from the optical transmission panel. That is, the optical output interruption alarm sent during the cold standby is not fetched but the optical output interruption alarm sent during the active state is stored. Thus, whether the optical output interruption alarm comes from a fault or the cold standby is discriminated and the system is switched automatically from any panel.