According to a distribution of image data values corresponding to individual spatial coordinate points on a three-dimensional space obtained when a region to be observed is tomographically taken by a radiographic medical diagnosis system, an interpolation area is provided within a segment set within a range of the image data values. In the interpolation area, degrees of chromaticity and opaqueness of each spatial coordinate point are made variable in response to the magnitude of image data value. The degrees of chromaticity and opaqueness of all the spatial coordinate points located on each line of sight connecting each plane coordinate point on a two-dimensional projection plane and a point of view are respectively integrated, and thus integrated values are reflected on the plane coordinate points on the line of sight.