Kobo Daishi Kukai (774-835) who settled Shingon Esoteric Buddhism in Japan is widely known Buddhist monk. He wrote many books with his remarkable talent in writing. Most of all, "Sangoshiki" that contains the comparison of Confucianism, Taoism and Buddhism and the determination of being Buddhist monk is worthy of notice. In this paper, opportunity to lead Kukai into becoming Buddhist monk is studied through "Sangoshiki". As a result of study, the following two results could be obtained. Firstly, he met one Buddhist monk and inherited "Gumonzihou" from him. These factors impact greatly on Kukai to become Buddhist monk. "Gumonzihou" was one of the Buddhist asceticism in vogue at that time. Kukai believed Buddhism as genuine truth through this asceticism and decided to be Buddhist monk. Secondly, another big factor impacting on Kukai to become Buddhist monk is his brother`s death. Kukai was absorbed in Buddhist impermanence and seek being Buddhist monk by his brother`s death. This was discovered through linguistic analysis of "Sangoshiki" preface and last volume of impermanence. Therefore, it can be known that "Sangoshiki" describe the process of being Buddhist monk in his own language. The contention of this paper is to reevaluate "Sango shiki" as the declaration of being Buddhist monk.