Unzen Volcano is located on the Shimabara Peninsula, the western extremity of a large graben across the Central Kyushu from NEE to SWW. Almost all lava domes and other volcanoes are distributed in the "Unzen Graben" caused by E-W trending faults such as Chijiwa fault. The Unzen volcanic rocks are characterized by biotite bearing hornblende andesite which belongs to the calc-alkaline rock series. They contain a large amount of cognate xenoliths. The activity of Unzen Volcano is divided into 4 stages; Stage I (Taka-dake volcanic unit), Stage II (Kusenbu-dake volcanic unit), Stage III (Mayu-yama volcanic unit), and Stage IV (Fugen-dake volcanic unit). The Chijiwa fault was formed between the stage III and IV. The Si02 frequency distribution diagram of the Unzen volcanic rocks shows a distinct high at 61-62 wt¾and the same is said of other "San-In" volcanic rocks. Furthermore, the diagram of the Stages I and II is quite different from that of the Stages III and IV. That is, the former has a peak at 61-62 wt%, whereas the latter shows an irregular distribution. The characteristics of crystallization of the Unzen volcanic rocks can be explained by the two assumptions; (1) plagioclase floated in the magma chamber, and (2) magnetite settled down so rapidly that no large phenocrysts of magnetite are found in the rocks