This study analyzes the communicative function of inferential adverb きっと(kitto). The inferential adverbs have some grammatical features that deeply related in morphologic rules with other elements of the sentences. In this reason, the former studies mainly discussed it with the grammatical point of view. But it has the limit to explain the concept of 'modality' in the inferential adverbs, so we use the pragmatic approach to this study. So we examine the principles of language usage reflected in the inferential adverb with a polite strategy point of view. We look at the process of the inferential adverb used as a tool to realize the politeness The study for the communicative function of きっと(kitto) suggests the pragmatic knowledge for the improvement of the communication skill that make us to manage language. And finally it will provide the valuable viewpoint that is available to the Japanese language education.