1. A medical image-processing apparatus that aligns volume data of two medical images, comprising: a setting part configured to set landmarks of medical images based on each set of volume data; a generating part configured to generate positional relationship information that indicates the positional relationships of said landmarks with regard to each set of volume data; a coordinating part configured to coordinate between the landmarks of two medical images that remain after eliminating some of the landmarks based on said positional relationship information; and an alignment part configured to align two sets of volume data based on said coordination of landmarks, wherein said generating part executes the processes of: forming multiple pairs, each of which comprises two landmarks of the same medical image based on the landmarks set by said setting part; calculating the distance between landmarks for each of said multiple pairs; and generating distance information containing said distances as positional relationship information; and wherein said coordinating part executes the processes of: tentatively coordinating the pairs that have a difference in distance that is a predetermined value or less by comparing the distance between a pair of landmarks of one of the medical images and the distance between a pair of landmarks of the other medical image based on the distance information; forming a group of three or more landmarks for each of the tentative pairs by adding a landmark that is not contained in the pairs; calculating an evaluated value indicating the degree of coordination of said group in the two medical images, based on the distance between the two landmarks contained in the group; and coordinating the landmarks between the two medical images based on the evaluated value.