It has long been a matter of debate whether language is a necessary, if not the only, criterion which distinguishes a nationality or a 'nation.' The present paper is an attempt to clarify the relation of language to nationality. First it is shown that the debate is due partly to the confusion of the terminologies such as 'nation,' and partly to the misunderstading of the nature of language. Second it is shown that there are two general propositions involved in the debates whether a nationality can be defined in terms of a language. The two proppositions can be formulated as follows: 'a group sharing a language is a nationality' and 'a nationality share a language.' Next, this paper demonstrates that the ralationship of language to nationality should be examined at three levels at least, that is, reality, perception and norm. The relatioship of a given language to a given nationality should be determined only after the examination in terms of the two propositions at three levels of reality, perception and norm.