1. An X-ray CT image taking apparatus for obtaining a reconstructed image from projection data obtained by X-ray image taking, provided with: calculation means for obtaining the degree of consistency of projection data opposite to the projection data of a projection angle at each projection angle; determination means for determining the effective region of said projection angle by the use of said degree of consistency obtained by said calculation means; and tomographic image forming means for reconstructing an image by the use of said projection data in said effective region determined by said determination means to thereby obtain a tomographic image.