An image generator which generates data of a plurality of images based on an output from the transmitter/receiver, a blood flow image generator which generates data of a plurality of blood flow images based on an output from the transmitter/receiver, a similarity calculator which calculates similarities between the plurality of images, a specifying processor which specifies at least two images exhibiting a low similarity from the plurality of images based on the similarities, an image selector which selects at least two blood flow images respectively corresponding to scanning times of the specified at least two images from the plurality of blood flow images and a display which displays the selected at least two blood flow images.
