It is an object of the present invention to eliminate windmill artifacts that inevitably occurs when a helical scan is performed with an X-ray CT system. To this end, the present invention is equipped with an X-ray source, and an X-ray detector having a plurality of detector elements arranged two-dimensionally, and disposed opposite to the X-ray source with a predetermined rotation center axis therebetween. The invention further includes reconstruction means for performing an arithmetic operation, in which two-dimensional projection data obtained based on X-ray detection data detected by the detector elements while rotating the X-ray source around the rotation center axis are back projected along a path different in a Z-axis direction extending along the rotation center axis from the X-ray path of the projection data, to reconstruct an image.