1. A forensic system configured to acquire digital information recorded on a plurality of computers or a server to analyze the acquired digital information, the forensic system comprising: a digital information acquiring unit configured to acquire digital information containing digital document information composed of a plurality of document files, and to acquire user information about users using the plurality of computers or the server; a recording unit configured to record therein the digital information acquired by the digital information acquiring unit; a display unit configured to display the recorded digital information; a user-specifying information setting unit configured to set user-specifying information showing which one of users contained in the user-specifying information each of the plurality of document files is related with, to set ranking information showing a first relative degree of relationship a first user from among the users has with litigation, and showing a second relative degree of relationship a second user from among the users has with the litigation, and configured to cause the recording unit to record the set user-specifying information, via the display unit; a user selecting unit configured to select at least one user via the display unit; a control unit, comprising: a searching unit configured to search a document file where the user-specifying information corresponding to the selected user was set, and to provide a book-mark function allowing book-mark search for material set with a hierarchy structure book-mark; a highlight display function configured to highlight, on the display unit, a searched word or phrase; a managing unit including an access right control function configured to set one or more rights for each of a plurality of accounts associated with a browser, wherein the one or more rights set by the managing unit includes a manager right associated with the browser; a statistical data producing unit configured to produce statistical data represented by data size for each data format of the acquired digital document information or statistical data represented by data size for each data format of the digital document information; a digital document extracting unit configured to select a kind of file to be searched; a data converting unit configured to preserve a selected file as a separate file; an additional information setting unit configured to set additional information showing whether or not the searched document file is related with the litigation, wherein the additional information includes at least one of a first tag indicating that the searched document file is related with the litigation, a second tag indicating that the searched document file is potentially related with the litigation, and a third tag indicating that the searched document file is not related with the litigation; and an output unit configured to output the document file which is related with the litigation, based on the first relative degree of relationship the first user from among the users has with the litigation, the second relative degree of relationship the second user from among the users has with the litigation, and the at least one of the first tag indicating that the searched document file is related with the litigation, the second tag indicating that the searched document file is potentially related with the litigation, and the third tag indicating that the searched document file is not related with the litigation.