A plurality of Doppler image data corresponding to a plurality of sliced planes of an object which are different from one another, respectively, are received. The plurality of Doppler image data shows distributions of first and second moving directions and moving velocities of a moving substance in the object on the plurality of sliced planes, respectively. First and second data showing distribution corresponding to each of a plurality of velocity ranges of the moving velocities among the distribution of the first and second moving direction, respectively, of the moving substance on each of the sliced planes is obtained based on the Doppler image data. Each of first and second three-dimensional image data is obtained based on the data corresponding to each of the plurality of velocity ranges and also corresponding to all of the plurality of sliced planes among all of the first and second data, respectively. Third three-dimensional image data is obtained by synthesizing all of the first and second three-dimensional image data and data corresponding to respective first three-dimensional image data and second three-dimensional image data among the third three-dimensional image show tones of color different from one another with three-dimensional image data corresponding to the third three-dimensional image data being displayed.