A path 22 representing the center line of a curved cylinder 21 is acquired (step S1). The path 22 can be set by a GUI while a volume rendering image displayed on a display device is viewed. Alternatively, the path 22 can be set automatically when the curved cylinder 21 is designated as a subject of observation. Then, a region of the curved cylinder 21 is extracted as a subject pf observation with the path 22 used as the center (step S2). Then, sections 23 and 24 are generated as if the extracted region was cut open along the path 22 (step S3). In this case, the sections 23 and 24 are curved along the curvature of the curved cylinder 21. Accordingly, CPR images are synthesized on the curved sections (step S4).