An apparatus and method for generating image data for preview images and displaying the preview images based on the generated image data. Each preview image corresponding to a respective page of document data having a plurality of pages. The displaying of the preview image based on the generated image data is started before the generation of the image data for the preview image for all the pages is completed. Then, an instruction of a process regarding at least one of the displayed preview images is accepted and the process is executed, before the displaying of all of the preview images is completed.