[en] In order to establish the scientific and technical basis of geological disposal of high level radioactive waste, Japan Atomic Energy Agency (JAEA) is advancing the geo-scientific research at the Mizunami Underground Laboratory (MIU). In this project, the surface-based investigation phase (Phase I) was finished in fiscal year 2004. Rock mechanics investigations were conducted in Phase I using the MIZ-1 borehole in order to understand the rock mechanical conditions deep underground and to construct a rock mechanical model of the MIU construction site. This report describes the results of these investigations. A brief summary is shown as follows. 1) Determining the average rock physical and mechanical properties using core from Toki Granite are: - apparent specific gravity: 2.62; - unconfined compressive strength (U.C.S.): 173Mpa; - tangent modulus at 50% U.C.S: 51.8Gpa; - poisson's ratio: 0.265. These results are consistent with the results of similar investigations in the nearby Shobasama site. 2) Estimation of in situ stress by stress measurements using cores. Four different methods (AE, DRA, ASR, and DSCA) were applied to estimate the in situ stress state. However the results are very low confidence due to very small deformation during drilling, excepting DSCA. 3) Determining of in situ stress by hydraulic fracturing. The horizontal, maximum principal stress is oriented to NW-SE (using north magnetic pole). Above six hundred meters depth, the in situ stress state is reverse fault type (S_H > S_h ≥ S_v overburden pressure ρgh as S_v). But below it, the in situ stress state changed to a wrench or normal fault type (S_V ≥ S_H > S_h). 4) Rock mechanical model. In consideration of the investigations and geological model, we proposed a rock mechanical model consisting of two in situ stress states and homogeneous, rock mechanical properties. (author)