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 the opposite side of the subject to the X-ray source and carries out helical scanning around the revolving axis and an image processor for creating a tomogram of the subject from the data collected by helical scanning using the X-ray detector, wherein the image processor creates a tomogram by reconstructing an image from data on the subject including a plurality of sets of projection data of different phases of helical scanning at the same point on the revolving axis and adding the reconstructed images of different phases, whereby a good image is created while suppressing aritifacts.