This paper is to study the position of a convention in the national law system of Japan. Firstly, in order to investigate Section 2 of Article 98 of the Japan Constitution , literatures which had been probably referred to during the establishment of the Japan Constitution were found out from historical facts occurring during the establishment of the same section of the Japan Constitution . An investigation was then conducted through a comparison of the related provisions of the Japan Constitution . In this process, the author frequently compared various theories to summarize earlier and later principles of logic and emulated a condition in which a relevant convention harmonizes the national laws. At last, the author attained his own hypothesis, i.e. a theory that the position of a convention is changeable with its moving stages. After examining the examples in which conventions are concluded and implemented in Japan, the author has a belief that the position stepwise changeable theory can be established. Specifically, before a desire to conclude a convention is brewed, the convention is not in contact with any national law. They are administered individually and are irrelevant to each other. But, once conclusion of a convention is desired, the convention will contact the national law. At this juncture, people of insight and authorized persons need to check the related national law and conduct a filtering task. But, once a stage of conclusion is entered, the essence of convention predominance will begin to manifest and will be more and more apparent. After a series of adjustments between the convention end and the national law end, the convention, in theory, will eventually equally harmonize and integrate with the national laws. That is to say, for the most part, the convention and the national laws are necessarily in a quasi-harmonization state. Nevertheless, once the convention contradicts the national laws, the essence of convention predominance will manifest once again. However it is defied, it is finally necessary to take every forcible harmonizing action to achieve a quasi-harmonization state again.