[en] In order to establish the scientific and technical basis for geological disposal of high level radioactive waste, Japan Atomic Energy Agency (JAEA) is proceeding with the geoscientific research in the research galleries excavated at the Mizunami Underground Research Laboratory (MIU). In this project, developing a conceptual model of the geological environment is one of the scientific and technical objectives. In the first step, the model was based on information from the surface-based investigations. Once excavation of research galleries began, the model could be improved using more detailed information from underground-based investigations. The investigation described here in is the measurement of the rock mechanical properties and initial stress of the rock mass around the GL.-200m level research gallery. A brief summary is presented as follows. 1) The average physical and mechanical properties of the rock mass determined using core from the Toki Granite on this level are: apparent specific gravity (dry): 2.62; unconfined compressive strength (U.C.S.): 143.2MPa; tangent modulus at 50% U.C.S: 51.9GPa; Poisson's ratio: 0.34. 2) Estimation of in situ stress using the core-based DSCA measurement method was applied to numerous samples from three boreholes. Estimated stress states are close to: σ₁:σ₂:σ₃=3:2:1 (for all results). σ₁=8.7 MPa in the NW-SE direction, σ₂=5.8MPa, σ₃=3.3MPa (data obtained near joint observed by borehole T.V. were omitted). 3) Determining in situ stress from stress relief (08MI14 borehole). σ₁:σ₂:σ₃=4:2:1. σ₁=10.6MPa and horizontal in the NW-SE direction, σ₂=5.2MPa, σ₃=2.6MPa. 4) Comparison of results at the GL.-200m level in the MIU with MIZ-1 investigation results: The average rock physical and mechanical properties at the GL.-200m level research gallery are consistent with the results from MIZ-1 testing. In situ stress measurements indicate that the stress state and stress level at GL.-200m depth match the observations in MIZ-1 at the same depth. This indicates that the surface based investigations in MIZ-1 are valid at this level. (author)