Volume data are collected by swing scanning in which frames in different field angle settings are mixed by a wide scan set at a wide field angle to image a diagnostic target and an index part for recognizing the position of the diagnostic target and by a narrow scan set at a field angle &thetas2 narrower than the field angle in the wide scan to image the diagnostic target with high time resolution. Then, the wide ultrasonic image is used to set spatial coordinates based on the index part. The spatial coordinates are used to align the narrow ultrasonic image. While this positional relation is being maintained, the wide ultrasonic image, the narrow ultrasonic image and a given ultrasonic image are displayed in a predetermined form.
