In this study, we analyzed the communication function of `adverbs of probability` in natural conversations between university students and between first meeting people with social experience, and in Japanese intermediate textbooks. Results are summarized as follows. The frequency of `tabun` is the highest in both conversations between university students and conversations between first meeting people with social experience. For communication function, there was no difference between `probability` and `mitigation` in conversations between first meeting people with social experience, but the ratio of `mitigation` was higher than `probability` in conversations with age difference. And the ratio of `mitigation` was higher than `probability` in conversation between university students. In other words, the use of `adverbs of probability` is influenced by the age and the degree of intimacy between speaker and talking partner. But, in Japanese intermediate textbooks, we could not investigate effects of the use of `adverbs of probability` because of no showing attribute of the conversation.