To provide an X-ray CT apparatus capable of reducing X-ray exposure and ineffective exposure by using half shadow without associated deterioration of image quality.

In the X-ray detector 22, the dimensions of the device are decided in order that the double width of unit slices (D2-Dx-1) of complete shadow is the width (D1, Dx) over which the half shadow at both sides of the complete shadow covers, and then CT images can be restructured by using the complete shadow and the half shadow. The dimensions are X-ray source to X-ray detector distance (SDD), Source-ISO center distance (SOD), Source-Collimator distance (SCD), Gantry Aperture (GA), Focal spot length (FSL) of the slice direction of X-ray source, unit slice thickness (SPC) at the center of the gantry, and unit slice thickness (SPD) in the X-ray detector.