The Obasute debris flow deposits, distributed in the Obasute area of Koshoku City, are produced by mass movements from older debris deposits. The older debris deposits form the depositional plane which ranges in altitude from 750 to 850 meters above sea level. Based on the stratigraphical relationship with the Omachi tephras distributed in this area, it is evidented that the age of older debris deposits is estimated as about 100, 000 years before present. The ages of the Obasute debris flow deposits were determined as 13, 500±460 yr. B.P. and 3, 250±260 yr. B.P. by radiocarbon dating. The fact indicates that the intermittent occurrences of twice mass movements are distinguished among the Obasute debris flow deposits.