The development of a geothermal reservoir evaluation method is mentioned for the purpose of reducing the risk of the geothermal development and drawing up an appropriate power generation plan. As for simulators, a geothermal reservoir simulator (SING-I,II) and a geothermal well two-phase simulator (WENG) are developed. For SING-II, made are evaluation of fracture effects in the pressure interference test, production predicting simulation using fracture models, development of a multicomponent gas-liquid two-phase simulator and a tracer simulation function. As to WENG, verification on fluids containing CO2 and survey on fluids containing NaCl are made. In the Mori area which is a model field, continued observation on reservoir pressure is conducted as well as temperature/pressure logging and chemical analysis of spouted fluids. By three-dimensional grid models of reservoirs, reappearance of natural conditions of temperature/pressure distribution and discharge into the ground surface can roughly be made, and history matching is tried to be made. Newly also in Oguni area, reappearance of natural conditions is simulated using SING-I. 13 refs., 189 figs., 46 tabs.