Emergency Rescue Information Sharing System supports rescue operations in early stage by gathering information about victims in need of help at the time of disaster. In this system, in-vehicle relays are used to gather extensive information. In-vehicle relays transmit information sent by mobile terminals to a database server via gateway switches. There are several wireless connections between relays and gateway switches to make the transmission redundant. However, information loss occurs when all links are disconnected. In addition, mobile terminals cannot send information without available wireless connections. In this paper, we propose a disaster information gathering mechanism that adopts Web-based offline operation and OpenFlow-based routing control. Offline operation of the system is realized by information caching and synchronization. Wireless links between in-vehicle relays and gateway switches are controlled by OpenFlow. Our mechanism selects a link for transmission dynamically in response to the status and priority changes of the wireless links. In order to prevent information loss while relays isolated from networks, information from mobile terminals is temporarily stored in in-vehicle servers. Prototype demonstration shows that the proposed mechanism can achieve high information gathering rate at the database server.