1. An image processing apparatus comprising: a first calculation unit for acquiring a moving image including a minor axis cross section of a carotid artery and a cross section of surrounding tissues around the carotid artery, estimating, from images of two temporally different frames of the acquired moving image, an optical flow of each point included in an area corresponding to a carotid artery wall and an optical flow of each point included in an area corresponding to the surrounding tissues, and calculating, based on the estimated optical flow of each point, an amount of displacement of the carotid artery and the surrounding tissues with respect to a radial direction of the carotid artery depending on a change in internal pressure of the carotid artery; and a second calculation unit for calculating an amount of displacement of the carotid artery and the surrounding tissues with respect to the radial direction of the carotid artery depending on a change in the internal pressure of the carotid artery based on a theoretical expression obtained by modeling the carotid artery and the surrounding tissues with a two-layer cylinder and calculating the elastic modulus of the carotid artery wall and the elastic modulus of the surrounding tissues so as to minimize a square error between the thus calculated amount of displacement and the amount of displacement calculated by the first calculation unit.