[en] For the safety assessment of HLW geological disposal system in a fractured host rock of such as granite, the homogeneous parallel plate model is usually applied as one of the nuclide migration evaluation models. The transmissivity coefficient and the fracture aperture as an average characteristic of fracture is one of important parameters used in the parallel-plate model. However, the natural fractures have complex characteristics such as infilling of gauge, branching and roughness of fracture surface. Therefore, the fracture aperture is distributed heterogeneously. The methodology how to justify those parameter values is one of the important issues, in case of modeling the fractures which have heterogeneous with the 1-dimensional parallel-plate model. As one of the studies for solving the issue, 50cm scale of the rock block includes single natural fracture was excavated from Kamaishi in-situ experiment site and observed a fracture by grinding the rock surface iteratively. In this study, a series of the digital image data of the fracture was obtained in 500 grinding sections, perpendicular direction to fracture profile. The coordinate of fracture center and the separation distance between the fracture surface were measured in intervals of 1mm at each section, and the fracture shape data was obtained about 250,000 data points. Thus, the average characteristics that the natural fracture of 50cm scale rock block, such as, the average of fracture separation distance, roughness of fracture surface, spatial correlation of fracture separation distance etc. was evaluated. Moreover, anisotropy of fracture permeability was presumed from these parameters. A CD-ROM is attached as an appendix. (J.P.N.)