The works of Miyazawa Kenji are full of fantastic descriptions and it may be the essential characteristic of his literature. The author discusses the origins of these fantastic characteristics from Miyazawa’s original literary method called ‘mental sketch’ and his innate sensitive trait. In the former aspects, Miyazawa had a view of the world which regarded all things were perceptive phenomena in the mind, and he wrote poems called ‘mental sketches’ from this viewpoint. Furthermore, he frequently recorded his ‘mental sketches’ during all-night ramble in the mountains and wilds or on the night train and ships. These situations probably made him experience supernatural phenomena which are the characteristics of his poems.In the latter aspect, Miyazawa was highly sympathetic to others from childhood and showed high hypnotizability in puberty. These episodes suggest the thinness of his ego-boundary. The author also discusses that the dissociative experiences which Shibayama pointed out in the Miyazawa’s works can be interpreted as the alteration in the ego-boundary. These sensory traits may also contributed to the fantasies of his works.