An image diagnostic system controls a probe to perform radial scanning within a body cavity, to acquire reflected signals through the probe and to produce data based on the signals. The system includes plural storage units for storing the data in line units, respectively, a writing control unit for controlling writing processing in accordance with a transmission/reception timing of the signals, and a reading control unit for controlling reading processing in accordance with rotation angles of the probe. The writing control unit writes data in the storage unit, other than the storage unit being subjected to reading processing, storing the oldest data. The reading control unit reads the data from the storage unit, other than the storage unit being subjected to writing processing, storing the latest data. The tomographic image is constructed based on the data read by the reading control unit.