A processor of an image processing apparatus designates a target region. When an image is divided into a plurality of small regions which are smaller than a target region, the processor calculates a centroid obtained by weighting the central coordinates of each of the plurality of small regions by a likelihood of each of the plurality of the small regions and calculates a covariance matrix that depends on the centroid, the likelihood being based on characteristic value histograms of respective small regions inside and outside the target region. The processor detection unit detects a degree of separation according to a histogram, a likelihood, a centroid, and a covariance matrix. The processor also determines a post-update target region according to a degree of separation.