An arbitrary cross section of an object is designated and a depth perpendicular to the designated cross section is designated. A cross-section projected-image data representing a cross-section projected-image obtained by projecting, onto a plane parallel to the designated cross section, averages of the pixel values arranged in the directions of depth in the region defined by the designated cross section and the designated depth including the designated cross section is generated. Image processing conditions are set on the basis of analysis of the cross-section projected-image data, Image processing is carried out on the cross-section projected-image data on the basis of the image processing conditions and an image is displayed on the basis of the processed cross-section projected-image data.