In order to reduce the amount of CO2 emitted to the atmosphere, injection of CO2 into underground is considered one of the useful approaches. Existing data of CO2 solubility in water suggests that hotter and deeper rock masses may also be candidates for CO2 underground sequestration due to enhanced CO2 solubility under high pressures even at high temperatures. To understand the initial reaction of several rocks in a CO2-saturated hydrothermal system, experiments were conducted using a batch-type over a range of 100-350°C up to 250 bars and numerical computations of phase equilibria based on the experimental results were carried out. The rocks used in our study were granite, sandstone and calc-silicate rock. Solution chemistry and examination of the rocks' surface texture showed that, in the CO2-saturated system, the release of alkali metals and alkaline-earth metals and the deposition of secondary minerals were promoted. According to EDX analysis and numerical phase equilibrium calculations, the secondary minerals formed might be diaspore, kaolinite, clay minerals, Ca-alminosilicates and calcite. However, there was no clear evidence for the calcite formation. It is implied that the precipitation rate of calcite is relatively slow and the formation of Ca-alminosilicates occur much easily than calcite even if solution is supersaturated with respect to calcite.