In Part I, the necessary conditions for the formation of typhoons are discussed. The author shows that the role of vertical thermal instability of the atmosphere may be the most important among many factors of formation in the light of current stability theory. Moreover, he enumerates the consequences of observation which assure the analytical evidences of the action of vertical instability. In Part II, the structure of typhoons and its schematic model are discussed. The author, at first, intends to expatiate the interpretation of the analytical results about the structure of the typhoon “ Ruth ” written in the previous paper (1953) by comparing the results of other analysis given by many authors. The salient features (§ 2, (1)-(4)) in its structure are almost coincident with those obtained by the author (1952) under theoretical considerations. Synthesizing the informations of the above results, of those written in the previous papers and the theoretical considerations, the author attempts to propose the schematic model about the structure of a mature typhoon which is consistent with the consequences of observations by aircraft and the radar photographs. Lastly, the mechanism of the formation of nonuniform cloud band in typhoons is discussed.