1. An image processing system comprising: processing circuitry configured to extract a mutually-same region of interest from each of a plurality of pieces of three-dimensional image data corresponding to mutually-different time phases; determine, on a basis of feature points included in the pieces of three-dimensional image data, a position used for superimposing together the regions of interest extracted from the pieces of three-dimensional image data, in a substantially same position of a subject; generate a plurality of parallax image groups that the pictures to be viewed change in accordance with shifting of the viewpoints of the viewer, by performing a rendering process on each of the pieces of three-dimensional image data while moving a point-of-view position by a predetermined parallactic angle, so as to include the regions of interest; hide a part of a region of interest to be on the outside so that a region of interest to be on the inside is exposed, when each of the regions of interest in the plurality of parallax image groups is superimposed; and display, on a display, a superimposed stereoscopic image formed by superimposing the plurality of parallax image groups, so as to superimpose the regions of interest together in the position determined.