2. The dialog supporting apparatus according to claim 1 , wherein said dialog history selection unit is further operable to calculate a degree of similarity between each of the dialog histories other than the extracted one or more of the plurality of dialog histories, by using as an utterance block, an assembly which corresponds to successive utterances and is commonly included in each of the dialog histories and the extracted one or more of the plurality of dialog histories, to determine degrees of similarity which are inversely proportional to a number of the utterance blocks or to determine degrees of similarity which are proportional to a mean of numbers of utterances included in utterance blocks, wherein the dialog history having the highest degree of similarity is selected, from the determined degrees of similarity, and from among the dialog histories of each of the dialog participants.
3. The dialog supporting apparatus according to claim 2 , wherein said dialog history selection unit is further operable to identify numbers of utterances included in respective assemblies, and calculate, by using the identified numbers of utterances, the degree of similarity which is proportional to a mean of the numbers of utterances included in the respective assemblies.