This paper discusses the validity of the topic word setting as a subclass of independent words. What this paper argues can be summarized as follows: First, the topic of information structure and the topic word of syntactic structure differs in the definition of the topic, reflection of the discourse context, direction of analysis, and essentiality. Second, topic words can be included in subclasses of independent words. Both the topic word and the independent word have a common characteristic of a sentence component that deviates from the argument structure of the verb. In addition, the existence of the presentational word in the system of independent word allows the topic word to be viewed as an independent word. Third, the topic word and the presentational word have a fixed position in the sentence, and represent different types of topic. Therefore, they should be included in each subclass of independent word.