An image determining unit determines a type of each of a plurality of images to be combined. A character identifying unit identifies a black character area in each of the images. A color-conversion processing unit performs a color conversion or a monochrome conversion on the images and a monochrome conversion on the black character area. When the images include a color image, a control unit controls the color-conversion processing unit to perform the color conversion on the images based on the type of the image and a current image processing mode. An image output unit combines color-conversion processed images on a single page and outputs combined images.