1. An image processing device comprising a nucleus-candidate-region extracting unit that extracts, from a captured image obtained by image-capturing a sample piece including a target cell having a nucleus, a nucleus candidate region corresponding to the nucleus; a basic-probability-information acquiring section that acquires, for each of a plurality of determination subject regions determined on the basis of the nucleus candidate region extracted by the nucleus-candidate-region extracting unit, basic probability information indicating probability that an image in the determination subject region is an image of the target cell, on the basis of a feature amount of the image of the determination subject region; and a probability-information calculating section that calculates probability information indicating probability that an image in a display subject region corresponding to the nucleus candidate region is the image of the target cell, on the basis of the basic probability information acquired for each of the plurality of determination subject regions, wherein the probability-information calculating section calculates, as the probability information indicating the probability that the image in the display subject region corresponding to the nucleus candidate region is the image of the target cell, an increasing function value that uses the basic probability information acquired for each of the plurality of determination regions as a variable, or an increasing function value that uses the basic probability information acquired for any of the plurality of determination regions and the number of the plurality of determination subject regions as variables.