Unzen volcano is located in Shimabara Peninsula (Nagasaki prefecture), Kyushu Island, southwest of Japan, one of the most active volcanoes within Japan. The most recent eruption occurred in 1990 to 1995. The possibility of existing or degenerating of a new hydrothermal system beneath Unzen volcano associated to the last eruption, was investigated by using repeat gravity monitoring since August 1999. The observed gravity changes depend significantly on changes in shallow groundwater level change. A good correlation of gravity with precipitation is observed with a phase lag about 3 months in (A) region, 4 months in (B) region and 3 to 4 months in (C) region. The water level change is discussed using Unzen Scientific Drilling Project porosity data. The results show that the downflow of permeating rain water is dominant.

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