Case assignment is one of the most important issues in theoretical lin-guistics. The goal of this paper is to describe both syntactic and semantic bases for case assignment in terms of PrototY.I.1~theory [1,4}. and to give an explanation for the difference in case assignment between English and Japanese by examining the Instrument Subject construction and deverbal nouns. English allows the structural case assignment under the syntactic prototype, while Japanese does not, because it has a strong requirement for the semantic prototype.