[en] The MIU ( Mizunami underground Research Laboratory ) will be a major deep underground research facility in Japan. One of the main goals of the MIU project is to establish comprehensive techniques for investigating the geological environment. An important part of the MIU project is to develop methodologies for modelling and groundwater flow simulation. In this study, we performed geological modelling and groundwater flow simulation to evaluate the influences of the hydraulic properties of a Tsukiyoshi fault and hydraulic boundary conditions on the hydrogeological environment and to define issues to be addressed in the future. In order to characterize the hydrogeological environment using groundwater flow simulation, investigations of the heterogeneous hydraulic properties of faults and the development of methods to establish hydraulic boundary conditions are essential. (author)