Computer networks comprise essential infrastructure in modern society and must function even in a disaster situation. Therefore, fault-tolerant networks are being actively studied. Disaster information systems, however, suffer from two main issues: lack of their utilization in peacetime and the difficulty for a non-expert to manage them should a disaster strike. Therefore, we place special emphasis on the development of a reliable network infrastructure that can function during both normal and disaster times, using a Wi-Fi-based wireless mesh network. In a large-scale disaster situation, our goal is to identify a way to reconstruct the mesh network by adding the minimum number of spare access points (APs) to ensure the reachability of all mesh routers to the backbone network. Furthermore, we consider that only public workers without any experience with wireless communication technologies must decide upon the adequate locations for spare APs and install them. Both of simulation experiments and field trial prove the effectiveness of the proposed methods.