1. An X-ray CT apparatus comprising: an X-ray data acquisition device which allows an X-ray generator, and a multi-row X-ray detector provided in opposing relationship to the X-ray generator and detecting X rays or a two-dimensional X-ray area detector of a matrix structure to be rotated about a center of rotation placed between the X-ray generator and the detector, thereby acquiring projection data of the X rays transmitted through a subject disposed between the X-ray generator and the detector; an image reconstructing device which image-reconstructs the projection data acquired from the X-ray data acquisition device; and a display device which displays an image-reconstructed image, wherein the X-ray data acquisition device is configured in such a manner that a detector channel width d 2 at each peripheral portion of the detector as viewed in a channel direction becomes d 1 <d 2 with respect to a detector channel width d 1 at a central portion thereof as viewed in the channel direction, or a plurality of detector widths (d 1 , d 2 ,. .. d i ,. .. d n-1 , d n ) provided from the central portion of the detector as viewed in the channel direction to the peripheral portion thereof satisfy d 1 ≦d 2 ≦. .. ≦d i ≦. .. ≦d n-1 ≦d n .