1. An operation method for processing data file, comprising the steps of: (a) displaying a reduced-size image for use in identifying contents of a data file, and a file icon associated with said reduced-size image in pair at a predetermined interval between them, said file icon having a smaller area than said reduced-size image; and (b) performing at least either one of the operations of i) selecting a function to be applied to said data file and ii) changing a display position of said reduced-size image by a drag-and-drop operation on said file icon.