This paper clarified the actual conditions of heavy rain disasters in Hiroshima Castle Town and its surrounding areas. Hiroshima suffered 33 torrential rains in the early Modern era, eight of which were floods that swept down a bridge in Hiroshima Castle Town. The Hiroshima clan worked on countermeasures for each flood. But there was no way to do that once in a decade. The slopes of the Hiroshima Bay also collapsed several times, but each time the farmers recovered and worked to expand production. The Modern era is called the Little Ice Age. Therefore, heavy rain disasters occur regardless of global warming.