1. A visualization apparatus, comprising: a memory; and a processor configured to use the memory and execute a process, the process comprising: setting, for each brightness value for voxels included in a predetermined Region Of Interest (ROI) in a three-dimensional volume data, opacity according to an appearance frequency of the brightness value; generating three-dimensional image data 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; and superimposing and displaying a cross section image generated from data of voxels on the cross section and the three-dimensional image data.