1. An image processing device, comprising: an obtaining unit configured to obtain a plurality of images captured in time series; a first calculating unit configured to calculate a first change vector indicating a change between the images in an angle representing a posture of a subject included in each of the images; a second calculating unit configured to calculate a second change vector indicating a change in coordinates of a feature point of the subject; a third calculating unit configured to calculate an intervector angle between the first change vector and the second change vector; and a determining unit configured to determine that the subject is three-dimensional when the intervector angle is smaller than a predetermined first threshold.