1. An image composition device comprising: a determination unit that determines an overlap state between respective pixel strings on corresponding lines in a plurality of first image data pieces, having a plurality of continuous pixels in the same color as a pixel string, including continuous coordinates with start point coordinate information and end point coordinate information of the pixel string, using the plurality of first image data pieces; and a composition unit that composes a plurality of second image data pieces based on a result of determination by the determination unit.