In modern Phonology, Godan verb, which is categorized by traditional grammer of Japanese Linguistics, is classified as a Consonant verb. Additionally, vowel endings of conjugation such as ren-youkei are regarded as suffix. However, there is a consonant verb like `kau(買う)`, whose base is finished with vowel. And this classification is often efficient in Japanese education, but it is difficult to give simple and consistent explanation on phonological phenomena such as onbin, as for example, we have to set some new rules according to each cases. Also, since onbin is a result of transition of Japanese language, it is not a best way to analyze it with sybchronic point of view. Some part should be explained with diachronic point of view. In a viewpoint of occurrence and development of onbin, It does not seem natural to regard simply a Godan verb, ren-youkei `-i` as an infix, if we consider the role of it. Also, a ren-youkei including Godan verb is widely used with the role of having nouniness. Therefore, it would reasonable to consider ren-youkei as a fundamental form or base of the verb.