1. An image processing device comprising: an image acquisition unit that drives an image sensor, acquires a first image by reading a surface of a medium exposed to a visible first light, and acquires a second image by reading a surface of the medium exposed to an ultraviolet second light; an edge image generating unit that applies an edge-extracting image processing filter to the first image and generates a first edge image, and applies the image processing filter to the second image and generates a second edge image; and a common-edge-removed second image generating unit that detects common edge parts where a first edge extracted in the first edge image and a second edge extracted in the second edge image are at corresponding positions, removes the common edge parts from the second edge image, and generates a common-edge-removed second image.