1. An image forming system including a data processing apparatus for creating document data for an image to be formed and displaying a preview image corresponding to the document data on a display section, and an image forming apparatus for forming an image based on data acquired, wherein the data processing apparatus comprises: an instruction receiving section for receiving an image formation instruction for the document data; an image data generating section for generating image data for the preview image when the instruction receiving section receives the image formation instruction; an image display section for displaying the preview image based on the image data generated by the image data generating section on the display section; and a formation data generating section for generating, based on the image data for the preview image, data for image formation which is to be sent to the image forming apparatus, and the image forming apparatus comprises: an acquiring section for acquiring the data for image formation from the data processing apparatus; and an image forming section for forming an image based on the acquired data for image formation.