1. An ultrasonic image processing apparatus, comprising: a memory configured to store volume data generated by ultrasonic scanning; a processing circuit configured to generate, from the volume data, data of a plurality of slice images corresponding to a plurality of slices; and generate, from the volume data using a predetermined viewing direction, data of a three-dimensional image; and a display configured to display the slice images and the three-dimensional image, wherein the processing circuit is further configured to set a vector on a first slice image of the displayed slice images in accordance with an instruction from an operator, the first slice image corresponding to a first slice of the plurality of slices; change a position defining a second slice of the plurality of slices and a position defining a third slide of the plurality of slices, the second slice corresponding to a second slice image of the displayed slice images, the third slice corresponding to a third slice image of the displayed slice images, so that the second slice includes the set vector indicating an extraction region and a viewing direction and is perpendicular to the first slice, and the third slice intersects the set vector at a predetermined point; and update the displayed three-dimensional image based on the set vector indicating the extraction region and the viewing direction, and update the second and third images based on the changed second and third slices.