Hydrological and thermal structures of the Doroyu area are investigated by using a numerical convection model. The model is two-dimensional and in steady state. The reservoir is confined by a cap rock of constant thickness on the top, heated by an impermeable interface at the bottom, and surrounded by impermeable interfaces at the both sides. The governing quasilinear partial differential equations in terms of stream function and temperature are solved by a finite difference technique. A comparison of the measured results with the calculated results is made about the temperature distribution in the reservoir. The relationship between the temperature distribution and the behavior of hot water within the reservoir is well explained. The average permeability of the reservoir composed of the lower Minasegawa and Doroyu formations is estimated as 5.28×10-3μm2.