File difference is managed. Group data and difference detection data tables are stored. A previous difference detection file, which is an original file of a difference management object file, is searched, and a difference between the previous difference detection file and the difference management object file is detected. Difference detection data is used to detect the difference, and is recorded on the difference detection data table. A group to which the difference management object file belongs is selected based on a group condition registered in the group data table. The difference is detected based on comparing extracted difference detection data and file data of the difference management object file. A set of the file data and the group is recorded as a new difference detection data on the difference detection data table, when the difference is detected.