1. An image processing device, comprising: a print image data generator that generates print image data for each of colors to be used to print a document image from image data of the document image, the image data including a barcode image comprising a barcode, and a background image overlapped on the barcode image as a background; a barcode region detector that detects a barcode region in the image data of the document image; a mask generator that detects a principal print color of the barcode image based on the print image data for each of colors in the barcode region, and generates mask data based on the print image data for the principal print color in the barcode region, wherein the mask data indicates a mask region occupied by a bar of the barcode image; and a density adjuster that reduces a density of pixels inside the mask region, by using the mask data, in at least any one of pieces of the print image data of colors for the barcode image.