Kuju volcano is situated in central Kyushu, Japan, which has a two-phase volcanic geothermal reservoir beneath the active fumarolic field. In October 1995, a phreatic eruption occurred near the fumarolic field of Kuju volcano. After that, active steam discharge activity has been continuing from the new craters and the pre-existing fumarolic field. We have been continuing to monitor the volcanic geothermal reservoir through the multiparameter measurements (including thermal, hydrological, gravimetric, magnetic, seismic, geodetic etc.). The mass balance in the volcanic geothermal reservoir and the cooling of the heat source concerned were discussed based on the multiparameter measurements, which shows that the hydrological state of the volcanic geothermal reservoir is reaching a new equilibrium state gradually and the temperature of the heat source is decreasing after the eruption. Monitoring by thermal, magnetic and gravimetric measurements are very effective to understand changes in the physical state of a geothermal reservoir, especially for phreatic eruption which is very similar to production without reinjection of geothermal fluids.

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