1. An X-ray CT apparatus comprising: an X-ray data acquisition device configured to acquire X-ray projection data transmitted through a subject positioned between an X-ray generator and a two-dimensional X-ray area detector configured to detect X-rays and positioned opposite the X-ray generator, the X-ray generator and the two-dimensional X-ray area detector rotatable about a center of rotation corresponding to a position of the subject; an image reconstructing device configured to reconstruct the projection data acquired from the X-ray data acquisition device to generate a tomographic image; an image display device configured to display a two-dimensional image extracted from a three-dimensional tomographic image continuous in a z-direction corresponding to a direction of travel of a cradle supporting the subject; an imaging condition setting device configured to set imaging conditions used for acquiring the projection data and displaying the tomographic image; and an image filter processing device configured to: define a pixel of interest in the two-dimensional image and at least one pixel neighboring the pixel of interest in the three-dimensional tomographic image, the at least one neighboring pixel positioned with respect to the pixel of interest in a direction orthogonal to the two-dimensional image; and perform an image filter processing for the pixel of interest in the two-dimensional image, wherein said image filter process varies depending on an image characteristic quantity of the at least one neighboring pixel.