「I am a cat」 made Natsume Soseki famous. This novel also has the way of Natsume's thinking about a human, an individual, a society and so on. In addition, His humor in this work is studied as his disposition. 「I am a cat」 made him a famous writer as well as a popular writer in Japan. Owing to that, 「I am a cat」 is studied by a lot of people. In this paper, the origin and the reason of this novel is considered. 「I am a cat」 is written in 1905(Meiji 38). This novel consisted of 11 times' serial novels on the Hototogisu from January 1905 to November 1906. In order to cure Nastume's neurosis, Takahama Kyoshi persuaded him to write down 「I am a cat」. In this novel, it is easy to find various kinds of his dissatisfaction, and he wrote down them as if he had been a cat. The success of this work made his health better than before. After this novel, he did never write novels like this although he was asked to write them, and he did never write humorous novels again. However, it is considered that 「I am a cat」 is the origin of his novels.