The purpose of this article is to identify the discourse functions of "ano" considered to be filler or hesitation marker. First, using the free conversational data, I specified by the discourse structure and the expressions used with "ano", the environment in which "ano" appears. The results revealed that the use of "ano" is divided in two groups as follows: 1) "ano" which appears when the speakers and hearers deal with problems in understanding the utterance; 2) "ano" which appears when the speakers provide the presuppositional information to avoid problems in hearers' understanding of the utterance. The former is related to the repair organization. From the viewpoint of repair, the latter has a common point with the former, since "ano" of both groups appears when the speakers insert the supporting segment for understanding the core segment in conversation. This paper suggests that "ano" contributes to marking the boundary between the supporting segments and the core segments.