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 processor, and a memory, wherein the memory has stored thereon instructions that, when executed by the processor, causes the processor to: cause a database to register 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; retrieve the keyword registered in the database from the document information; extract a sentence including the retrieved keyword from the document information; classify the extracted document information based on the determination result for each determination result, and analyze and select the keyword that appears commonly in the classified document information; calculate a score indicating a degree of relevance to the lawsuit based on a feature value extracted from the extracted sentence; and change a degree of highlighting of the sentence according to the score, wherein the degree of relevance is assigned to the keyword using a document information group for which it is determined by the user that the document information group is related to a lawsuit in the past; wherein the database registers the keyword when the degree of relevance exceeds a predetermined threshold value; and wherein the feature value is extracted based on an appearance frequency or an amount of transmission information related to the keyword.