A printing system includes a data processing device and an image forming apparatus. The data processing device processes print data. The data processing device includes a data receiving unit, a data managing unit, a data transmitting unit, and a preview image generating unit. The preview image generating unit generates a preview image based on print data. When the data receiving unit receives print data of a plurality of pages, the preview image generating unit automatically generates the preview image of a part of pages among the plurality of pages. The data managing unit manages the preview image generated by the preview image generating unit. When the data managing unit manages the preview image of a page requested from the image forming apparatus, the data transmitting unit transmits the preview image managed by the data managing unit to the image forming apparatus.