1. An image processing device, comprising: an image acquisition part acquiring image data from a document; a character detector detecting data of a character area, which includes at least a character therein, in the image data acquired from the document and outputting a character detection result; a marking detector detecting data of a marking area, which is an area where a mark is drawn by a user, in the image data acquired from the document and outputting a marking detection result; and an image processing part, based on the character detection result and the marking detection result, performing a non-readable process, which makes the character in the character area non-readable, to the data of the character area so as to create modified image data that is different from the image data acquired from the document and is to be used for an image forming process when the data in the character area is covered by the marking area and the non-readable process is determined necessary for making the character non-readable, not performing the non-readable process to the data of the character area when the data of the character area is not covered by the marking area.