1. An ultrasonic image processing apparatus comprising: a storage unit configured to store volume data generated by ultrasonic scanning; a slice image generation unit configured to generate data of a plurality of slice images corresponding to a plurality of predetermined slices based on the volume data; a three-dimensional image generation unit configured to generate data of a three-dimensional image associated with a predetermined viewing direction based on the volume data; a display unit configured to display the slice images and the three-dimensional image; a setting unit configured to set a vector on a first slice image of the displayed slice images in accordance with an instruction from an operator; an updating unit configured to update the displayed three-dimensional image based on the set vector; and a changing unit configured to change a position of a slice of a remaining slice image of the displayed slice images to a position where the slice intersects the set vector and is perpendicular to the slice of the first slice image.