Main constituent clay minerals in the Oashizawa specimens are smectite, mica/smectite mineral and chlorite/smectite mineral. The former two minerals occur with small amounts of mica, kaolinite and chlorite. It is clear from the mineralogical properties that the mica/smectite mineral was formed from smectite by adding alkaline cation and increasing temperature. This transformation starts by increasing the alkaline cation content and smectite changes to mica/smectite mineral with g=0 in the early stage. The mineral formed transforms to mica/smectite mineral with g=1 by encreasing temperature and pressure. The minerals with g=1 occur mainly in relatively lower parts of the Kusanagi Formation. Chlorite/smectite mineral can be formed from montmorillonite by adding Mg and Fe cations, but the genesis is not certain for the mineral found in the lowest part of the Kusanagi Formation.