1. An image processing apparatus that determines a priority ranking of a subject detected from an image, the image processing apparatus comprising: a detection unit that detects one or more subjects from an image; a computation unit that computes a priority for determining a priority ranking, for each of the subjects detected by the detection unit; and a determination unit that determines the priority ranking of each of the subjects detected by the detection unit, based on the priority computed by the computation unit, wherein the computation unit: assigns a larger weight to a subject detected by the detection unit at least one of when the subject has a larger size and/or when the subject is positioned nearer a center of the image; corrects the assigned weight by a larger weight when a most recent priority ranking determined for the subject by the determination unit in a previous image from which the subject is detected is higher; computes a higher priority for the subject when the corrected weight is larger; and, when a predetermined condition is satisfied, computes the priority with a reduced amount of correction by the weight of the most recent priority ranking, and wherein the computation unit obtains an amount of change between the image and the previous image used when determining the most recent priority ranking and, when the amount of change is determined to be not smaller than a predetermined amount, computes the priority with the reduced amount of correction by the weight of the most recent priority ranking.