In this visualization method, for each brightness value for voxels included in a predetermined Region Of Interest (ROI) in a three-dimensional volume data, opacity is set according to an appearance frequency of the brightness value. Then, three-dimensional image data is generated for a portion on and under a cross section set for the three-dimensional volume data, by using color data that corresponds to a brightness value of each voxel included in the ROI and the opacity of each brightness value. Then, a cross section image generated from data of voxels on the cross section and the three-dimensional image data are superimposed and displayed.