In an ultrasonic image processing apparatus, an original image in the form of a three-dimensional ultrasonic image is generated from volume data based on the volume rendering method. Directional interpolation processing is then applied to the original image. More specifically, a corresponding point corresponding to a noted pixel in the display image is first determined on the original image. Then, on the original image, a plurality of candidate directions extending through the corresponding point are set, and a dispersion value is computed for each of the candidate directions. Further, a normal direction with the maximum dispersion value is specified among the candidate directions, and a reference direction along the contour of a tissue is determined in the direction orthogonal to the specified normal line. Alternatively, the reference direction is determined as a direction with the minimum dispersion value among the plurality of candidate directions. Based on a plurality of pixel values existing in the reference directions, an interpolated pixel value of the noted pixel is computed. Contrast emphasis processing is further applied following the directional interpolation processing, as required.
