The basis of an assurance argument must be built on top of explicit specification of the target system. Nevertheless, identification of a municipal disaster management system out of existing documents is a non-trivial task. We propose an approach applying 6W1H models. A 6W1H model is a tree of actions equipped with “6Ws” (Who, What, Whom, When, Where, Why) that provide necessary explication of the system for assurance argument. The approach is exemplified by identifying a system and building an assurance case out of water supply activities prescribed in the Local Disaster Management Plan of Hiratsuka city.