To easily change the range of attention in an ultrasonic diagnostic system for synthesizing and displaying a narrow-range image with high resolution in the range of attention on a wide-range image as the background.

The position and posture of an ultrasonic probe 10 at the time of preparing the wide-range image and at the time of preparing the narrow-range image are detected respectively by a magnetic sensor and a magnetic field generator 12 built in the ultrasonic probe 10. Then, the wide-range image and the narrow-range image are positioned by a three-dimensional coordinate converting part 22 based on the position and posture at both times above, and the images are synthesized. Accordingly, the position of the narrow-range image in the wide-range image can be changed by such an easy operation in the system as changing the position and posture of the ultrasonic probe 10.