To properly set a standard axis serving as the standard of volume operation on the maximum cross section of an object.

An arbitrary cutting surface is set within a three-dimensional space and the arbitrary tomographic image 32 expressing it is displayed. A temporary axis 44 is set on this image. Tomographic images 34 and 36 for expressing two reference images as the orthogonal faces passing both end points 52 and 54 thereof are displayed and two target points 56 and 58 are indicated on those images. The direction of the temporary axis 44 is set as the axis passing those target points. Thereafter, the length of the temporary axis 44 is corrected. By this constitution, the standard axis is set on the maximum cross section of the object.