An image diagnostic system comprising: a radial scan motor; a rotary joint; a rotary encoder; a wavelength-swept light source; a display unit; a rotatable probe configured to be rotatably mounted by said rotary joint and rotationally driven by said radial scan motor positionable in a body cavity and configured to repeatedly transmit signals into the body cavity and receive signals reflected by biotissue surrounding the body cavity, wherein the rotation angles of the probe are detectable by said rotary encoder; a control unit configured to produce data based on the reflected signals to construct a tomographic image of the body cavity and surrounding biotissue, the control unit comprising: a line memory unit comprising plural line memories configured to have data written therein; a writing control unit configured to control writing processing of the data written in the line memories; a reading control unit configured to control reading processing of the data written in the line memories; wherein the probe is adapted to be connected to said light source capable of outputting light, with the probe being configured to transmit and receive the light, and to produce the data based on interference light between light reflected in the body cavity and received through the probe and a reference light split from the light outputted from the light source; and wherein the display unit is configured to display the tomographic image constructed by the control unit based on the data read by the reading control unit; characterized in that the writing control unit is configured to control the writing processing to write the data in the line memory in which the stored data is oldest; the reading control unit is configured to read the data written in the line memories in accordance with a rotation angle of the probe, and to control the reading processing to read the data in the line memory in which the stored data is newest; the plural line memories are adapted to receive the interference light data in cycle units of wavelength sweep of interference light, and the reading control unit is adapted to read the interference light data in accordance with output pulses from the encoder. a radial scan motor; a rotary joint; a rotary encoder; a wavelength-swept light source; a display unit; a rotatable probe configured to be rotatably mounted by said rotary joint and rotationally driven by said radial scan motor positionable in a body cavity and configured to repeatedly transmit signals into the body cavity and receive signals reflected by biotissue surrounding the body cavity, wherein the rotation angles of the probe are detectable by said rotary encoder; a control unit configured to produce data based on the reflected signals to construct a tomographic image of the body cavity and surrounding biotissue, the control unit comprising: a line memory unit comprising plural line memories configured to have data written therein; a writing control unit configured to control writing processing of the data written in the line memories; a reading control unit configured to control reading processing of the data written in the line memories; wherein the probe is adapted to be connected to said light source capable of outputting light, with the probe being configured to transmit and receive the light, and to produce the data based on interference light between light reflected in the body cavity and received through the probe and a reference light split from the light outputted from the light source; and wherein the display unit is configured to display the tomographic image constructed by the control unit based on the data read by the reading control unit; a line memory unit comprising plural line memories configured to have data written therein; a writing control unit configured to control writing processing of the data written in the line memories; a reading control unit configured to control reading processing of the data written in the line memories; wherein the probe is adapted to be connected to said light source capable of outputting light, with the probe being configured to transmit and receive the light, and to produce the data based on interference light between light reflected in the body cavity and received through the probe and a reference light split from the light outputted from the light source; and wherein the display unit is configured to display the tomographic image constructed by the control unit based on the data read by the reading control unit; characterized in that the writing control unit is configured to control the writing processing to write the data in the line memory in which the stored data is oldest; the reading control unit is configured to read the data written in the line memories in accordance with a rotation angle of the probe, and to control the reading processing to read the data in the line memory in which the stored data is newest; the plural line memories are adapted to receive the interference light data in cycle units of wavelength sweep of interference light, and the reading control unit is adapted to read the interference light data in accordance with output pulses from the encoder.