An operation device which displays a document preview image on a paper sheet preview image, includes: a changing unit which changes a size of document image data corresponding to the document preview image in accordance with a movement amount and a movement direction when a user touches a part of the document preview image and moves the touched part; a determining unit which determines whether or not a vertical size and a horizontal size of the document preview image after the change are larger than a vertical size and a horizontal size of a largest paper sheet preview image; and an enlargement continuous shooting setting unit sets an enlargement continuous shooting mode if it is determined that the vertical size and the horizontal size of the document preview image are larger than the vertical size and the horizontal size of the largest paper sheet preview image.