1. An X-ray CT apparatus comprising: a scanner that includes an X-ray source and an X-ray detector having two-dimensionally arranged X-ray detector elements, disposed on an opposite side of the X-ray source interposing a subject to be examined therebetween, and provided for measuring X-rays irradiated from the X-ray source onto the subject and passed through the subject and carries out helical scanning by rotating the X-ray source and the X-ray detector relative to the subject around a revolving axis and moving the subject relative to the X-ray source and the X-ray detector along the revolving axis; and an image processor for creating a tomogram of the subject from projection data collected by helical scanning using the X-ray detector, wherein the image processor creates a tomogram by reconstructing data combining a plurality of different sets of projection data at a point on the subject on the revolving axis such that phase ranges of the rotation overlap with each other.