Along with Nouns, Verbs and Adjectives, Japanese possesses another major lexical category, Nominal Adjectives. This paper examines the structure of the Nominal Adjective category, and shows that the category constitutes a prime example of a prototype category, exhibiting prototype effects in the syntactic behavior of its members. After demonstrating that the semantic factor of gradability motivates the syntactic behavior of Nominal Adjectives, the paper takes a diachronic perspective and examines how the category's prototype structure came about. Drawing on supporting historical evidence, the paper argues that Nominal Adjectives first emerged as a result of the metaphorical interpretation of container-based locational expressions. It also contends that a gradual semantic shift in the semantic structure of the category's members, from designating "things" to designating "properties", and their corresponding occurrence in the modifying function, are responsible for the current prototype structure of the category. (Satoshi Uehara)