1. An image processing apparatus comprising: a storage unit configured to store a plurality of volume data acquired by imaging a predetermined part of an object, the plurality of volume data corresponding to a plurality of phases; a calculation unit configured to calculate a motion vector of each voxel included in each volume data by performing registration between the plurality of volume data; an image generation unit configured to generate an image representing a local motion of a diagnosis part using the motion vector of each voxel, the local motion of the diagnosis part being indicated in a predetermined set of direction components of the motion vector, one of predetermined different colors and luminances being assigned to a corresponding one of the direction components in accordance with a magnitude of the corresponding one of the direction components; and a display unit configured to display the image representing the local motion of the diagnosis part.