1. An X-ray CT apparatus having a rotary unit for causing an X-ray source and an X-ray detector for detecting X-rays from the X-ray source to rotate integrally around a subject, comprising: a full data reconstructing device for reconstructing tomograms on the basis of penetrating X-ray data of each view detected during a full turn around said subject; a segment data reconstructing device for reconstructing tomograms on the basis of penetrating X-ray data of each view detected during rotating at a prescribed angle less than a full turn around said subject; and a change-over device for counting the tomograms reconstructed after the start of scanning, and changing over between said full data reconstructing device and segment data reconstructing device according to the count.