A method of displaying a thumbnail image of the original image of a data file stored in a storage unit on a display unit includes: (a) storing in the storage unit a first compressed code relating to an image for displaying the thumbnail image of the data file, the first compressed code being generated by dividing the image into a plurality of tiles and performing discrete wavelet transform and hierarchical encoding on the pixel values of the image tile by tile; (b) setting the resolution of the thumbnail image in accordance with the format type of the data file; (c) extracting a second compressed code according to the resolution that was set from the first compressed code stored in the storage unit; and (d) displaying the thumbnail image based on the second compressed code that was extracted.