[en] For the buffer of geological disposal of High-level radioactive waste (HLW) in Japan, it is expected to maintain its low water permeability, self-sealing properties, radionuclides adsorption retardation properties, thermal conductivity, etc. It is considered that compacted bentonite or a compacted sand-mixtured bentonite that satisfy many of the expected properties mentioned above are superior. JNC (Japan Nuclear Fuel Development Corporation) has studied the measurement method for the properties of buffer and measured to use the measurement results for the design and performance assessment analysis. Also, we have conducted the design of engineered barrier and underground facility based on assuming geological condition. For the design of engineered barrier and underground facility, high thermal conductivity of buffer is design requirement to avoid mineralogical alternation. Also, the design is not conducted using the density of buffer less than the bulk density (powder-mass density). Therefore, the bulk density (powder-mass density) is one of the design requirements. In this report, the thermal properties and the bulk density (powder-mass density) of the buffer material is measured. Then thermal analysis in the near field is conducted using the measurement results, and we studied the relationships between the dry density, sand-mixtured ratio, water content and thickness of the buffer to satisfy the design requirement from the view point of thermal condition, based on the temperature constraint of the buffer. (author)