A region extraction unit extracts first and second shape-invariant regions corresponding to each other from first and second three-dimensional images acquired by an MRI apparatus. A first registration unit acquires a first deformation vector by performing rigid registration between the first and second shape-invariant regions. A second registration unit acquires a second deformation vector by performing non-rigid registration between the first and second three-dimensional images in each shape-invariant region. A magnetic field distortion vector calculation unit calculates a magnetic field distortion vector, which represents relative magnetic field distortion between the first and second three-dimensional images based on the first and second deformation vectors.