A dialog supporting apparatus is provided which can support an on-going dialog so that the dialog is smoothly completed irrespective of the other party in the dialog. The dialog supporting apparatus includes utterance receiving units which receive the utterances of the dialog participants and output utterance information for identifying an utterance. Moreover, the dialog supporting apparatus includes utterance outputting units which output the utterance information from the other participant in the dialog as an utterance, and an utterance processing unit which transforms the utterance identified by the utterance information into an other utterance form. Furthermore, the dialog supporting apparatus includes a dialog history selection unit which selects an optimum dialog history in a dialog history database and an utterance prediction unit which predicts utterances in the dialog based on the selected dialog history.