PURPOSE: To obtain the tomographic image data of a sliced face having reliability in the case of interpolation processing of the detection data from an X-ray detector by moving a top plate in correspondence to the rotating speed of a gantry by a top plate movement control mechanism in accordance with the output of a detector for the rotating speed of the gantry.

CONSTITUTION: The rotating speed of the gantry 1 is detected at all times by the speed detector consisting of a rotary encoder 7 and the top plate 11 is moved in correspondence to the rotating speed of the gantry 1 by the top plate movement control mechanism 10 in accordance with the output of this speed detector. The moving speed of the top plate 11 is eventually set to follow up the rotating speed of the gantry 1 if this rotating speed decreases. The same holds true even when the rotating speed of the gantry 1 increases. The correspondence of the detection data successively delivered from the X-ray detector and the position of the top plate 11, i.e., the position of the sliced face of a testee body is, therefore, extremely exact. Then, the tomographic image data of the sliced face having the reliability is obtd. in the case of the interpolation processing of the detection data from the X-ray detector.