1. A data processing apparatus for generating image data for preview image corresponding to document data of a plurality of pages, for each page, and displaying preview images based on the generated image data, comprising: an image display section for starting a preview image display based on the already-generated image data before the image data generation for all the pages is completed; and an instruction accepting section for accepting an instruction of a process for the preview image before the preview image display for all the pages is completed, wherein the process concerning the instruction accepted by the instruction accepting section is executed before the preview image display for all the pages is completed.