A free-moving piece (jellyfish sign) having a high probability of separation in the near future in an unstable plaque is clearly displayed. Ultrasonic tomographic images (25) of a cross section including an axis of the carotid artery are successively obtained. Using a threshold value which sets a surface (48) of the unstable plaque as a boundary between brightness and darkness, the plurality of ultrasonic tomographic images (25) are binarized. A difference is calculated between corresponding pixels of two binarized ultrasonic tomographic images so that a free-moving piece (52) is extracted and displayed overlapped over the ultrasonic tomographic image (25).