This paper focuses on the features of the Japanese adjective “tashika-da” and the differences between two types of adverb “tashika-ni”. The adjective “tashika-da” changes its form to “tashika-na” as an adnominal, “tashika-da” as a predicate, and “tashika-ni” and “tashika” as an adverb, according to the function used in the sentence. When acting as an adjective, “takashi-da/na” has the original and lexical meaning which means /sure, certain/. The adverb “tashika-ni”has two types of usage. The one is called adverb of manner and the other is modal adverb. An adverb of manner “tashika-ni” occurs in front of a verb phrase and describes an event of the sentence in more detail, maintaining its lexical meaning of “surely, certainly”. On the other hand, a modal adverb “tashika-ni” is placed in the beginning of the sentence modifying a speaker’s mood, whereas it loses the lexical meaning /surely, certainly/ and the function of agreement works stronger. And also, a modal adverb “tashika-ni”can be used as a one word, expressing speaker’s agreement on a listener’s judgement.