An analysis conducted by NHK has found that about 47 million people, or nearly 40 percent of Japan's population, live in areas that face the risk of flooding.
NHK carried out the analysis based on flooding levels estimated by local governments.
Local governments have been asked by the central government to publish maps of estimated flood levels. The maps show projected inundation areas and water depths in the event of flooding caused by amounts of rain expected once in 1,000 years.
NHK made its own nationwide hazard map based on flood mapping data collected from the state and Japan's 47 prefectures. It also compared the map with census population figures.
The results showed that as of 2015, about 47 million people across the country were living in areas vulnerable to flooding.
Professor Nozawa Chie at Meiji University, who specializes in urban planning, cooperated in the analysis.
She said while there have been a series of flooding disasters, many people are unaware of the risk of flooding, and that they need to look at the hazard maps to understand the risks of where they live.
Nozawa pointed out that there are not enough evacuation shelters in areas where the population is increasing. She advises people to consider beforehand where to evacuate in order to safeguard their own lives.