1. A forensic system that acquires digital information recorded in a plurality of computers or a server and analyzes the acquired digital information, comprising: a database that registers a keyword for determining by a user whether a plurality of pieces of document information included in the digital information is related to a lawsuit; a retrieving unit that retrieves the keyword registered in the database from the document information; a sentence extracting unit that extracts a sentence including the retrieved keyword from the document information; a score calculating unit that calculates a score indicating a degree of relevance to the lawsuit based on a feature value extracted from the sentence extracted by the sentence extracting unit; and a highlighting unit that changes a degree of highlighting of the sentence according to the score.