The present Lake Onuma was formed by as intensive eruption and partial collapse of the volcanic cone in 1640 AD, which caused a debris flow that dammed the drainage. Three cores were recovered from Lake Onuma to examine the environmental change in the region. These three cores are deposits Ko-a, Ko-c1, Ko-d, and possible Ko-c2 were successfully identified with the help of radiocarbon age dating,137Cs radioactivity measurement, and chemical analyses, to construct an age model of these cores.