1. An X-ray CT scanner comprising: a gantry for collecting projection data about a patient; a reconstruction portion for reconstructing multislice image data or volumetric image data from said projection data at a given matrix size; a storage portion for storing said 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 said stored image data into a matrix size corresponding to said user's instruction and varying an image slice thickness of said storage image data into an image slice thickness corresponding to said user's instruction.