A wide-area evacuation plan has been formulated for the Koto Delta region because of the large population in the inundation area and the expected shortage of evacuation centers. However, carrying out wide-area evacuation as planned may be challenging due to multiple issues such as large-scale traffic congestion. Therefore, it is expected that numerous people will be isolated and their health will deteriorate due to long-term flooding. We propose an efficient rescue strategy to reduce the number of residents who are isolated for long periods of time. Therefore, we changed the conditions for rescuing isolated people by boats, such as the order of rescuing the isolated people and the evacuation rate, and estimated the changes in the number of isolated people and the time required for rescue completion under each condition. Next, we compared the estimated results under each condition to develop efficient rescue methods for isolated people.