Image diagnostic apparatus

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 interference light data based on the signals, the data being received in cycle units of wavelength sweep of interference light. The system includes plural storage units for storing the data, a writing control unit for controlling writing processing, 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.