An ultrasonic image is captured by an ultrasonic probe. A reference image is obtained by extracting a tomographic image corresponding to the scan plane of the ultrasonic image from volume image data that is pre-obtained by a diagnostic imaging apparatus and that is stored in a volume-data storing unit. The ultrasonic image and the reference image are displayed on the same screen. In this case, of the reference image, a portion corresponding to the view area of the ultrasonic image is extracted and the resulting reference image having the same region as the ultrasonic image is displayed as a fan-shaped image.
