1. An image processing apparatus comprising: a motion detection unit detecting a motion of a subject to be evaluated by using an image of the subject to be evaluated; a correlation calculation unit calculating a temporal change correlation between motion amounts of a plurality of portions of the subject to be evaluated, by using a motion vector indicating the motion of the subject to be evaluated, which is detected by the motion detection unit; and an evaluation value calculation unit calculating an evaluation value to evaluate cooperativity of the motion of the subject to be evaluated by using the correlation calculated by the correlation calculation unit, wherein the motion detection unit divides the entire region of the image of the subject to be evaluated into a plurality of partial regions and detects the motions of the respective partial regions, wherein the correlation calculation unit calculates the correlation between the partial regions using the motion vector calculated for each partial region by the motion detection unit, wherein the evaluation value calculation unit calculates the evaluation value by using the correlation between the partial regions calculated by the correlation calculation unit, and wherein the evaluation value calculation unit includes a correlation normalization unit normalizing the correlation between the partial regions calculated by the correlation calculation unit to a predetermined function, a distribution calculation unit calculating a time direction distribution of the correlation between the partial regions calculated by the correlation calculation unit, a distribution normalization unit normalizing the distribution calculated by the distribution calculation unit to a predetermined function, and an average/evaluation value calculation unit calculating, as the evaluation value, a ratio of the number of products, which are products of the correlations between the partial regions normalized by the correlation normalization unit and the distribution normalized by the distribution normalization unit, having a value larger than a predetermined threshold value to the total number of products.