PURPOSE: To instantaneously display an objective part by a three-dimensional (3-D) image while changing observation time.

CONSTITUTION: Plural 3-D data are previously constructed from plural two-dimensional (2-D) image data groups stored in an external storage device 4 and stored in a 3-D data area 3a of a memory correspondingly to data acquiring time. When observation time (time acquiring data to be observed) is applied, the stored 3-D data are extracted from the area 3a and 3-D image data are generated and stored in a display data area 3b. A 3-D image is displayed based upon the 3-D image data stored in the area 3b. Thereby a secular change in an affected part or the like can be instantaneously observed by 3-D images.