The purpose of this paper is to clarify the nature of Case ‘ni’ in modern Japanese from a syntactic point of view. In particular, I propose that issues left unresolved in previous studies can be resolved using a new syntactic test. In the previous studies, the study of Case ‘ni’ has been spotlighted by approaches such as syntactic, semantic, and pragmatic methods, drawing on the various uses of ‘ni’. Determining the nature of Case ‘ni’ in syntactic studies, especially which usage is grammatical or adverbial, is a very important issue in terms of case assignment. However, such a syntactic test has not been done. Therefore, it has been left as a future subject in previous studies. This paper is based on two observations.: (i) There is a strong syntactic constraint that NPIs in multiple NPI constructions in Japanese must always appear in the adjunct position and (ii) NPI ‘sika’ can be attached to various case particles. In other words, if ‘ni + sika’ can co-occur with other NPIs in the same clause, then the Case ‘ni’ is categorized as an adverbial Case, and if it cannot co-occur, it is classified as a grammatical Case.