An X-ray CT (computed tomography) scanner includes a gantry for collecting projection data about a patient, a reconstruction portion for reconstructing multislice image data or volumetric image data from the projection data at a given matrix size, a storage portion for storing the reconstructed image data, an input portion for entering a user's instruction regarding magnification or demagnification of image, and an image processing portion for converting the matrix size of the stored image data into a matrix size corresponding to the user's instruction and varying the image slice thickness of the stored image data into an image slice thickness corresponding to the user's instruction.