Clay minerals such as smectite, kaolin mineral and pyrophyllite and secondary minerals such as pyrite and gypsum are frequently found in fine fraction of volcanic ash from many volcanoes in Japan. Natural occurrences of clays and secondary minerals disclose geological and geochemical settings and alteration environment. Chemical composition and mineral assemblage of alteration products in volcanic ash can provide authentic information on chemical conditions inside volcanoes and their temporal variations indicate change in chemical condition beneath volcano before and after its activation. Researching assemblage of clay minerals in volcanic ejecta and their chemical compositions is a safety method for analysis of volcanic activity and will contribute to prediction of change in volcanic activity.