To obtain a tomographic image suffering little from artifact and the determination of the quality of image.(1) Step 1: With the axis of abscissas as the fan angle or channel direction of an X-ray detector and the axis of ordinates as the projection angle direction of an X-ray source 11, projection data are arranged side by side to thereby prepare a sinogram.(2) Step 2: from the sinogram, the projection data of an opposite fan beam is prepared at each projection angle.(3) Step 3: the degree of consistency with the projection data of the opposite fan beam is obtained at each projection angle.(4) Step 4: The region of reference error of the degree of consistency is obtained, and a projection angle region is determined from this region of reference error.(5) Step 5: the projection data in the projection angle region obtained at the step 4 is reconstructed to thereby prepare a tomographic image.