1. An image processing device which obtains a corresponding vector indicating an association from a target block on a first image to a corresponding block on a second image, the device comprising: an acquisition unit configured to acquire one or more candidate vectors from among corresponding vectors which have been calculated for each of one or more candidate blocks around the target block; an arrangement unit configured to arrange one or more particles indicating positions on the second image around each candidate corresponding block indicated by each of the acquired one or more candidate vectors while using the target block as an origination, and arranging one or more particle vectors indicating the one or more particles, respectively, while using the target, block as an origination; a calculating unit configured to calculate a correlation, between a pixel value of the target block and a pixel value of each block defined by each of the one or more particles, and giving a weight depending on the calculated correlation to each of the particle vectors; and a processing unit configured to obtain the corresponding vector for the target block based on the weight calculated for each of the one or more particle vectors.