1. A dialog supporting apparatus which supports an on-going dialog made by a first dialog participant and a second dialog participant, said apparatus comprising: a recording medium for storing a plurality of dialog histories each comprised of dialogs which are utterance strings of a plurality of people including the first dialog participant, the second dialog participant, and one or more dialog participants other than the first dialog participant and the second dialog participant, the dialog histories being stored in association with first information identifying the people, including the first dialog participant, the second dialog participant, and the one or more dialog participants other than the first dialog participant and the second dialog participant, of the dialog histories and second information indicating a topic of a dialog; a dialog history selection unit operable to: obtain the first information identifying the first dialog participant and the second dialog participant and the second information indicating the topic of the dialog; extract, from said recording medium, one or more of the plurality of dialog histories based on the first information identifying the first dialog participant and the second dialog participant and the second information indicating the topic of the dialog, wherein each of the extracted one or more of the plurality of dialog histories is associated with a corresponding first dialog participant or second dialog participant indicated by the obtained first information and the topic of the dialog indicated by the obtained second information, and each of the extracted one or more of the plurality of dialog histories is either (i) a dialog history of a dialog made between the first dialog participant and a third dialog participant other than the second dialog participant or (ii) a dialog history of a dialog made between the second dialog participant and a fourth dialog participant other than the first dialog participant; and select, from said recording medium, a dialog history that is different from any of the extracted one or more of the plurality of dialog histories, wherein the selected dialog history has a highest degree of similarity with respect to the extracted one or more of the plurality of dialog histories; and an utterance prediction unit operable to predict a next utterance of either the first dialog participant or the second dialog participant in the dialog, based on the dialog history selected by said dialog history selection unit, and to predict, as a next utterance of either the first dialog participant or the second dialog participant in the dialog, an utterance which is included in the dialog history selected by said dialog history selection unit and appears next to an utterance corresponding to an utterance in the dialog.