An image diagnostic system controls a probe to perform rotational and axially-moving scanning within a body cavity and to acquire reflected signals through the probe. Based on the acquired reflected signals, the system forms and outputs tomographic image of the body cavity and biotissue surrounding the body cavity. The system is provided with a detection unit which detects a direction of movement of the probe in the axially-moving scanning and a control unit which controls the rotational speed of the probe in the rotational scanning. The control unit includes a determination unit which determines whether or not the rotational speed of the probe is to be changed upon detection of movement of the probe toward the distal direction within the body cavity, and changes the rotational speed of the probe upon determination of a change at the determination unit.
