1. An image processing device comprising: an image acquirer configured to acquire a time series of images from each of a plurality of imaging devices; a tracker configured to track a position of a target object in the time series of images; a storage configured to store distributions of feature values for respective regions into which the time series of images is divided depending on a distance from a corresponding imaging device; an extractor configured to extract an observed value corresponding to a feature value in each of the regions from the time series of images; a feature amount acquirer configured to acquire a feature amount based on a deviation value of the observed value in the distribution for the corresponding region; and an association controller configured to associate target objects tracked by the tracker with one another among multiple time series of images acquired from the plurality of imaging devices by using the feature amount.