Abrupt and large rises in well water levels were observed three days before the 2000 eruption of Usu volcano, simultaneously with the increase of seismic activity. After the groundwater level in the DT1 well rose 4.07 m, the groundwater discharged one day before the eruption. The groundwater level rose 0.95 m in the DT2 well before the eruption. These water-level rises are caused by crustal deformation from magma migration, and enable us to estimate 0.63-11.2 ¿ 10-6 of compressional volumetric strain by applying the wells' responses to earth and ocean tides. These strain changes are explained by a 2.8--16.5 ¿ 106 m3 spherical inflation source model located at the Usu volcano. A 5 ¿ 106 m3 inflation source model is estimated by a horizontal displacement observed by GPS near DT2, which is consistent with the volumetric strain estimated by the water-level rises in DT2.