1. A file search system for searching for a file stored in a storage device, comprising: reception means for receiving a search request made by a user; search means for obtaining search result files by searching the storage device on the basis of notification from the reception means; monitor means for monitoring a file operation by the user; creation means for creating a reference relation of the search result files on the basis of the file operation by the user monitored by the monitor means, the reference relation being defined as a relation between a reference file and a reference source file in which information included in the reference file is introduced into the reference source file by the file operation of the user, wherein the reference file is a file to be referenced and the reference source file is a file referencing the reference file, and the introduction of information from the reference file to the reference source file is performed by copying information from the reference file and pasting the copied information into the reference source file; storage means for storing the reference relation; importance calculation means for calculating an importance of the reference file on the basis of one or more items selected from among reference relation items each obtained by monitoring a file operation of the user regarding introduction of information from the reference file to the reference source file; and search result sort means for arranging the search result files obtained by the search means on the basis of the importance of the reference file; and a control section for displaying a screen for allowing the user to select arrangement of the search result files obtained by the search means on the basis of the importance or a condition other than the importance, the control section judging whether or not the user selects the arrangement of the search result files on the basis of the importance; wherein the search result sort means arranges the search result files on the basis of the importance when the arrangement of the search result files on the basis of the importance is selected, and arranges the search result files on the basis of the condition other than the importance when the arrangement of the search result files on the basis of the importance is not selected.