A method for structuring a communications network based on an asynchronous transfer mode in which communications are held with the use of cells transferred between a node (C) on the upstream side and a node (D) on the downstream side. The method for structuring the network includes a step for preparing a plurality of VPI conversion tables for converting VPIs of the cell input under normal communicating conditions and that of the cell input in the event of a failure and outputting the cell with the VPI converted. The prepared VPI conversion tables are reorganized for each node and the reorganized VPI conversion tables are distributed to all nodes. Then, an alternate route monitoring and switching virtual path (15, 16) is set up in each alternate route and a monitoring cell is periodically transmitted along the alternate route.; When a failure occurs, the VPI conversion table is switched over according to the pattern of the failure and the input VPI is converted in accordance with the switched VPI conversion table, and thereby, an alternate virtual path is set up in the relevant alternate route.