Though his ideas and thoughts originated from Zhuzi (朱子) himself, ironically, Ito Jinsai’s intellectual fame came from his anti-Zhuzi stance, in a period of iron fist rule which was based on the ideology of Zhuzi. While Zhuzism is anti-utilitarianism, Ito Jinsai became more and more of a utilitarian until the end of his life in the year 1705. In Jinsai’s ideas and thought system, there were many mutually inconsistent parts. This article shows that these mutually inconsistent parts could be closely compared to those between Confucius and Mencius. Until his last day, he still could not solve this inconsistency problem himself. This article analyzes why Ito Jinsai could not solve this problem. Some say Confucius was a utilitarian while Mencius was an anti-utilitarian. Jinsai looked up to Confucius’ teaching with the highest regard and respect. He also saw Mencius as the most perfect interpreter of Confucius, and he did not differentiate between them. There may be two reasons for this. Firstly, he was afraid of the then extremely strict rule of the 5th generation Shogun - [Tokugawa, Tsunayoshi] – a believer in Zhuzi. Secondly, he still could not escape from the trap of one of Chinese Cultures biggest traditions, which closely bonded Confucius and Mencius together, without any inconsistencies.