A memory stores image data generated by sensors included in an imager and containing a component data piece for each of color components that has undergone A/D conversion by an A/D converter. A width detector detects, based on the image data having undergone image processing by an image processor, the width along the main scanning direction of a duplicate area in which pieces of image data generated by adjoining sensors overlap each other. A displacement detector detects, based on the width of the duplicate area along the main scanning direction, a displacement of a scan target object for each of optical systems included in the imager. A first blur corrector performs blur correction on the component data piece using a point spread function for each of the color components that is dependent on a displacement of the scan target object. An adjuster adjusts, based on a transfer magnification dependent on a displacement of the scan target object, the size of an image for each of the color components indicated by the component data piece. A combiner combines the images by superimposing portions of the component data pieces.