1. An image processing device for determining whether to carry out color printing or monochrome printing to achieve a printing operation on a print sheet based on an original document, the device comprising: a determination area obtaining unit configured to automatically obtain a partial area of the original document estimated to be printed on the print sheet; and a color/monochrome determination unit configured to carry out the determination with using image data of the obtained partial area, wherein the determination area obtaining unit obtains the partial area estimated to be printed on the print sheet based on a size of the print sheet, and wherein, in a case where the printing operation is carried out with setting a scaling factor for enlargement or reduction, the determination area obtaining unit obtains, as the partial area estimated to be printed on the print sheet, an area having a size obtained by dividing the size of the print sheet with the set scaling factor for enlargement or reduction located at a predetermined position of the original document.