1. An X-ray CT system comprising: an X-ray data collection device that causes an X-ray generator and a two-dimensional X-ray area detector, which is opposed to the X-ray generator in order to detect X-rays and structured with detector elements arranged in the form of a matrix, to rotate about a center of rotation located between them, and collects projection data items produced from X-rays transmitted by a subject lying between them; an image reconstruction device that reconstructs images according to the projection data items collected by the X-ray data collection device; and an image display device that displays the reconstructed tomographic images, wherein: the image display device detects a reference point in each of tomographic images, measures a deviation of each tomographic image in an x direction that is a horizontal direction and a deviation thereof in a y direction that is a vertical direction by measuring the deviations of the reference point, compensates the deviations, and displays resultant tomographic images.