1. A forensic system that acquires digital information recorded in a plurality of computers or servers and analyzes connection of the acquired digital information to a lawsuit, comprising: at least one processor acting as: a result information receiving unit that receives result information which is a determination result of connection between the lawsuit and a document group including a predetermined number of documents, which is extracted from document data included in the digital information, by a user; an element selection unit that calculates evaluation values of elements which commonly appear in the document group in each result information item from characteristics of the elements and selects the elements on the basis of the evaluation values, arranges the elements in a descending order of the evaluation values, extracts elements repeatedly until a sum of the evaluation values of the elements reaches a unique target value, and selects the extracted elements for improving an accuracy of the review process by the forensic system; a score calculation unit that calculates a score of each document in the document data from the selected elements included in each document of the document data and the evaluation values of the selected elements; a recall ratio calculation unit that calculates a recall ratio related to the determination of the connection to the lawsuit on the basis of the score; an element re-selection unit that re-selects the elements using the sum of the evaluation values of the elements and the difference between the unique target value and the recall ratio when the recall ration is less than the unique target value and extracts elements from a set of elements except for the elements, which have been extracted from the elements until the sum of the evaluation values of the elements reaches the difference between the unique target value and the recall ratio, until the recall ratio is greater than the unique target value, and selects the extracted elements; and the score calculation unit further calculates a second score of each document in the document data, using the elements selected by the element re-selection unit and the evaluation values of the elements selected by the element re-selection unit, and composes the scores and the second scores to re-calculate the score of each document in the document data, when the recall ratio is less than the unique target value, the score calculation unit further calculates a second score of each document in the document data.