1. An image processing device, comprising: an image reading part that reads an image on a document and generates image data based on the image; an input part that accepts an input designation of a division number into which the image data of the document generated by the image reading part is divided, and an output resolution of output image data created based on the image data; a division processing part that creates divided image data by dividing the image data of the document by the division number accepted by the input part; a controller that sets a reading resolution based on the division number and the output resolution of the output image data; and a data output part that outputs the divided image data as the output image data, wherein the controller controls the image reading part to read the image on the document at the set reading resolution, the controller determines the reading resolution to read the image based on the division number and the output resolution, and sets the determined reading resolution as the set scan resolution, the controller calculates the reading resolution by multiplying the output resolution by a square root of the division number, the controller compares the calculated reading resolution and a plurality of preset read executable resolutions at which the image reading part is able to execute reading, and the controller sets, as the reading resolution, the smallest one among the read executable resolutions that are greater than the calculated reading resolution when the calculated reading resolution does not match the read executable resolution.