An optical data transmission apparatus which is used to transmit a signal between the rotating portion and fixed portion of an X-ray CT scanner comprises light emitting elements arranged on a side plane of the rotating portion which is perpendicular to a rotation axis and are uniformly driven according to transmission data to emit lights in a direction parallel to the rotation axis. A light receiving element is disposed on the side plane of the fixed portion which faces the light emitting elements. The interval between the light emitting elements is so set that the illumination areas formed by the light emitting elements partly overlap each other on the light receiving element. Therefore, the light receiving element always receives a light or lights from one or two of the light emitting elements during the rotation of the rotating portion and transmission data from all of the light emitting elements can be continuously transmitted to the light receiving element.