1. An X-ray computed tomography (CT) apparatus comprising: a scan device for exposing X rays to a subject while at least one of a gantry and a table is being moved along a body-axis direction of the subject, to create projection data of the subject; a CT value change determining device for determining an amount of change in CT value in the body-axis direction between a plurality of tomographic images obtained by backprojecting the projection data, with respect to each of pixel areas contained in the tomographic images; an image processing condition selecting device for estimating a range of an amount of change in CT value caused by an artifact, and for selecting an image processing condition for performing an image process for reducing artifacts based on whether the determined amount of change in CT value falls within the estimated range of the amount of change in CT value caused by an artifact, wherein the estimated range includes the amount of change in CT value smaller than that estimated to be caused by a structure of the subject and larger than that estimated to be caused by noise within the tomographic image; and an artifact reducing device for performing an image process using the image processing condition selected by the image processing condition selecting device.