It is considered that the prototype theory and the metaphor theory can explain the semantics system of a polysemy successfully in the flied of cognitive linguistics. Based on those theories, this paper will describe and analyze every meaning of the Japanese kanji “内” and give a cognitive explanation to the relationship between the meanings to build the network model about the Japanese kanji “内”’s semantics system. In that semantics system, the prototype meaning is “inside”, the extension meanings are abstract concepts, time concept, condition concept, process concept, scope concept and pronoun. The two meanings are related by metaphor.