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The whole total distribution map of the volcanic ash fall deposit in Japan was summarized with the data base under the program of the "Research on volcanic ash fall hazard assessment and risk management for industrial location" and the "Impact analysis on the volcanic ash-fall in the metropolitan area". The heavy ash fall will cause the serious disaster of course, but well developed area also suffered heavy economical damage by even a thin deposit of volcanic ash. Around five hundred distribution maps of the volcanic ash fall deposits in Japan in these three hundred thousand years were collected. The first work is to redraw the isopach map completely for each unit, because major of the isopach map in the literature were not perfect in shape. The redrawing for each contour line was carried out mathematically to extend each semicircular curve to make a close circle automatically. The redrawn isopachs were reserved as a digital data and the thickness of the deposit between each contour line for around each one kilometer mesh, which is authorized by the third Digital National Land Information system in Japan, were calculated by logarithmical proportional allotment. The worked area was from 24 to 46 degree in the north latitude and from 123 to 149 degree in the east longitude. The name of the volcano, unit name and its alphabetical abbreviation, age of the eruption, and the references, of each ash fall unit were summarized as a data base for each mesh. So it is possible to show the thickness of each ash fall unit for each one kilometer mesh. For example Haneda airport and Narita airport were estimated to be suffered the 8.7 and 3.2 centimeters ash fall from Fuji volcano in 1707 respectively. And also it is easy to find the ash fall disaster history for anywhere and any municipality, cities and towns, which number is around two thousand, in Japan. For example the municipal office building of the Tokyo Metropolitan in Shinjuku has been suffered three, five, and seventeen times air fall ash in these one thousand, ten thousand, and a hundred thousand years, respectively, and the thickness of the deposit for each periods are also able to be shown using the data base.

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