1. A dialog supporting apparatus which supports an on-going dialog made by dialog participants, said apparatus comprising: a dialog history selection unit operable to select a dialog history from a dialog history database, based on information identifying the dialog participants and a situation of the dialog, the dialog history database storing dialog histories of the dialog participants and people other than the dialog participants; and an utterance prediction unit operable to predict a next utterance of one of the dialog participants in the dialog, based on the dialog history selected by said dialog history selection unit.