PURPOSE: To obtain a projection density by center projection method, which corresponds to a new viewpoint, even if a viewpoint position is changed.

CONSTITUTION: More than one interpolation CT picture is prepared in the middle of adjacent CT dislocation surfaces (flow 1). The picture is displayed and an observer sets the new viewpoint by viewing the display content (flow 3). The number of use sheets is decided in accordance with a distance between the new viewpoint and a projecting surface (flows 3 and 4). The interpolation CT picture is included in the number of use sheets. The picture of the number of the sheets is projected on the projecting surface and it is displayed. Then, a processing pitch is decided by the inclinations of a center projecting line and a CT dislocation image from the new viewpoint to the projecting surface, and interpolation is executed, if necessary, for the respective processing points.