When a reduction in the transmission speed of the line preset as the TDM line transfer route is detected by a device-connecting line termination unit, a mapping request, to which information indicating the amount of TDM-line bandwidth overflowing from the line and a destination indicating the final line relaying device have been added, is generated by the mapping request processing unit, and transmitted via the other line to another device connected in a ring network in order to map the overflow amount of bandwidth added to the mapping request on each line sequentially from the other device to the line relaying device as the final transfer destination, thereby forming a TDM-line detour route.