An image forming device capable of forming index images that correspond to a plurality of files, respectively, onto a recording medium includes an image data generating unit configured to generate one of first image data and second image data as data for forming the index image corresponding to each of the plurality of files, a file selecting unit configured to select, from the plurality of files, files for which at least one parameter related to file processing is to be detected, a parameter detecting unit configured to detect the at least one parameter for each of the selected files, a parameter judging unit configured to judge whether the at least one parameter as detected satisfies a predetermined condition, and a control unit configured to control the image data generating unit to generate one of the first image data and the second image data based on the judgment.