An image processing device according to an aspect of the present invention includes: at least one memory storing a set of instructions; and at least one processor configured to execute the set of instructions to: extract a cluster image including a cluster of particles from a particle image; estimate particle density of the cluster image; determine numbers based on the particle density, and synthesize a second dictionary image from first dictionary images having densities each being equal to the numbers, the first dictionary images being images of particles of which positions is obtained; match the second dictionary image with the partial image, and determine positions of particles in the cluster image based on a result of the matching and the positions of particles of the first dictionary images; and output the determined positions of particles.