The invention reduces an overhead operation required for re-establishment of a physical signal link. A physical signal link (layer 1 link) is switched over keeping a layer 2 link in the OSI reference model. A database 11 stores information indicating physical signal links and information indicating the maximum length of a layer 2 frame determined according to the ability of physical signal links. When a physical signal link is to be switched over, a target physical signal link is selected and a physical signal link used by the link is switched to the target physical link without releasing the single layer 2 link. The maximum length corresponding to the target physical signal link is obtained from the database 11, and layer 2 frames having the maximum length are transmitted. Switching over a physical signal link is performed in the following manner. When a physical signal link switching-over request is given, establishment of a target physical signal link is required without releasing a layer 2 link, and a multi-reception of control signals using both of the target physical signal link and an original physical signal link is started. After confirming establishment of the target physical signal link, a physical signal link used for transmitting a control signal is switched to the target physical signal link and release of the original physical signal link is required. After confirming release of the original physical signal link, a physical signal link for receiving a control signal is switched to the target physical signal link.