If failure is detected in multiple layers, a failure detection recovery processing unit is activated simultaneously in each layer and performs operations up to immediately before switching a main signal. The first layer to reach that stage notifies the other layer of this fact through inter-layer failure recovery information communication terminal and processing units to stop the failure recovery operation of the other layer. The other layer sends back a response to it through the inter-layer failure recovery information communication terminal and processing units, and the layer that first reaches the stage up to immediately before actually switches the main signal based on the result of the response.