This invention provides an X-ray CT apparatus capable of addressing a change in heart beats during a scan and a method of controlling the same. The X-ray CT apparatus includes: a data acquisition device for acquiring projection data of a subject by a helical scan synchronized with a heart beat; an image reconstructing apparatus for reconstructing an image on the basis of the collected projection data; and a controller for controlling the data acquisition device and the image reconstructing apparatus. The controller includes: first control means for changing helical pitch in accordance with a change in the heart beat during the helical scan; and second control means for reconstructing images in a plurality of slice positions at equal intervals on the basis of projection data also including projection data collected during the change in the helical pitch.