An image diagnostic apparatus for controlling a probe, which is adapted to be connected to the image diagnostic apparatus via a rotary joint, the probe being adapted to be rotationally driven by a radial scan motor coupled to a rotary encoder which is configured to detect a rotation angle of the probe, the probe being adapted to repeatedly transmit signals into a body cavity which are configured to be reflected by biotissue surrounding the body cavity to perform radial scanning within the body cavity, the image diagnostic apparatus comprising: a control unit configured to produce data based on the reflected signals and to construct a tomographic image of the body cavity and the biotissue surrounding the body cavity on a basis of the data; and a display unit configured to display the tomographic image, the control unit comprising: plural storage units in which are to be storable the data in transmission/reception units; a writing control unit configured to control writing processing of the data in the storage units in accordance with a transmission/reception timing of the signals; a reading control unit configured to control reading processing of the data stored in the storage units; the writing control unit configured to control the writing processing to write the data in the storage unit which contains stored data that is oldest and which is not being subjected to reading processing; the reading control unit configured to control the reading processing to read the data in the storage units which contains the stored data that is newest and which is not being subjected to writing processing; and the tomographic image being constructed based on the data read by the reading control unit, wherein the plural storage units are line memories (801, 802, 803, 1801, 1802, 1803, 2401, 2402, 2403), characterized in that the reading control unit is configured to control reading processing of the data stored in the storage units in accordance with a rotation angle, detectable by the encoder (214, 1506, 2206), of the probe configured to be rotatably mounted by the rotary joint (211, 1503, 2203) and rotationally driven by the radial scan motor (213, 1505, 2205) so as to perform appropriate reading of signal data in accordance with the rotation cycle of the probe during radial scanning even when synchronization does not exist between the rotation cycle of the probe in the radial scanning and the signal transmission /reception cycle of signals from the probe. a control unit configured to produce data based on the reflected signals and to construct a tomographic image of the body cavity and the biotissue surrounding the body cavity on a basis of the data; and a display unit configured to display the tomographic image, the control unit comprising: plural storage units in which are to be storable the data in transmission/reception units; a writing control unit configured to control writing processing of the data in the storage units in accordance with a transmission/reception timing of the signals; a reading control unit configured to control reading processing of the data stored in the storage units; the writing control unit configured to control the writing processing to write the data in the storage unit which contains stored data that is oldest and which is not being subjected to reading processing; the reading control unit configured to control the reading processing to read the data in the storage units which contains the stored data that is newest and which is not being subjected to writing processing; and the tomographic image being constructed based on the data read by the reading control unit, plural storage units in which are to be storable the data in transmission/reception units; a writing control unit configured to control writing processing of the data in the storage units in accordance with a transmission/reception timing of the signals; a reading control unit configured to control reading processing of the data stored in the storage units; the writing control unit configured to control the writing processing to write the data in the storage unit which contains stored data that is oldest and which is not being subjected to reading processing; the reading control unit configured to control the reading processing to read the data in the storage units which contains the stored data that is newest and which is not being subjected to writing processing; and the tomographic image being constructed based on the data read by the reading control unit, wherein the plural storage units are line memories (801, 802, 803, 1801, 1802, 1803, 2401, 2402, 2403), characterized in that the reading control unit is configured to control reading processing of the data stored in the storage units in accordance with a rotation angle, detectable by the encoder (214, 1506, 2206), of the probe configured to be rotatably mounted by the rotary joint (211, 1503, 2203) and rotationally driven by the radial scan motor (213, 1505, 2205) so as to perform appropriate reading of signal data in accordance with the rotation cycle of the probe during radial scanning even when synchronization does not exist between the rotation cycle of the probe in the radial scanning and the signal transmission /reception cycle of signals from the probe.