In order to elucidate the heat treatment effects on water vapor adsorption properties of Noto-diatomaceous earth (Noto-DE), the specific surface area (SSA) was measured with N2 and H2O, and pore size distribution and water vapor adsorption of the samples were also measured. Particles ＞75 µm and ＜ 2 µm in size were separated from the samples of Noto-DE and they were calcined at various temperatures up to 1,000°C. Both particles ＞75 µm and ＜2 µm showed that the SSA values measured with H2O were larger than those with N2. This may be caused by the chemical adsorption action on silanol radical of diatomaceous shell and the interlayer of clay minerals. Concerning samples after particle-size classification, fraction having particle size not greater than 2 µm showed the most water vapor adsorption. This result indicates that the clay minerals that account for ca. 40% of Noto-DE have a great influence on water vapor adsorption properties. Furthermore the reason why water vapor adsorption of Noto-DE decreased sharply at heat treatment above 800°C may be due to change of structure of montmorillonite. Therefore it was found that the changes in the water vapor adsorption properties of Noto-DE by heating were caused by the changes in the crystalline structure of montmorillonite.