4. The topic transition analysis system according to claim 1 comprising: at least one of topic similarity calculation unit calculating a topic similarity between the statement and a media stream at and around the time when the statement is made and a topic similarity between the statement and a group of statements made immediately before the statement, wherein the statement trigger string determination unit determines a string having a higher topic similarity to be a string that has triggered the statement.
7. The topic transition analysis system according to claim 4 , further comprising: a speech recognition unit generating a text string from a speech signal included in the primary media stream, wherein said at least one of the topic similarity calculation unit calculates a topic similarity between the statement and a text string generated by the speech recognition unit from the entire section or an arbitrary section of the media stream which is a similarity calculation target.