In order to construct a deductive legal knowledge base, it is necessary first to clarify the structure of the law as a deductive system from which a legal judgement can be justified as a conclusion of logical deduction together with relevant facts. As the legal state of affairs changes according to the time progress of an event, a clarified logical model of law is necessary to enable us to deduce changes among legal relationships over time from the beginning to the end of a case. This study presents such a model based on Logical Jurisprudence, in which the relationship between legal sentences and the legal meta sentences regulating the validity of legal sentences plays a definitive role. The model is applied to the United Nations Convention on Contracts for the International Sale of Goods (CISG) to develop a deductive knowledge base. The deductive structure of the contract law is clarified so that appropriate answers are deduced to questions about legal state of affairs at any time point as a results of the application of CISG provisions to a concrete case.