1. A system for searching texts for keywords, the system comprising: a first index storage unit for storing a first index which designates lists of keywords contained in the texts from identifications of the respective texts; a second index storage unit for storing a second index which designates lists of texts containing the keywords from identifications of the respective keywords, in a way that the second index corresponds to the number of texts containing the respective keywords; an input unit for receiving an input of a text search condition which is a condition of texts to be search targets of the keywords; a determination unit for calculating an estimation of search time by the first index and an estimation of search time by the second index, based on the number of texts satisfying the text search condition among the plurality of texts to be search targets, and based on the number of the texts stored in a way that the number corresponds to the second index, and for determining which one of the first and second indexes makes a search faster; and a search unit for searching for and outputting keywords which appear in texts satisfying the text search condition with higher frequency than a predetermined standard, by using the index which is determined to make the search faster.