4. The system according to claim 2 , wherein the search unit searches a plurality of texts corresponding to the section where the first section is excluded from the second section, while setting, as the keywords, words having the appearance frequencies that are different between texts corresponding to a first event and a second event to a larger extent than the other words.
4. The system according to claim 2 , wherein the search unit searches a plurality of texts corresponding to the section where the first section is excluded from the second section, while setting, as the keywords, words having the appearance frequencies that are different between texts corresponding to a first event and a second event to a larger extent than the other words. 5. The system according to claim 4 , wherein the search unit calculates an evaluation value of each word, the evaluation value taking a larger value according to the increasing rate of the number of appearances in the second section to that in the first section and taking a larger value according to the difference between the frequencies, and makes a search while setting words with larger calculated evaluation values as the keywords.