An X-ray computed tomographic apparatus includes a gantry unit having a detector having X-ray detection element rows along a slice direction and an X-ray tube and acquiring projection data from a subject by helical scanning, an extraction unit extracting first and second projection data sets corresponding to first and second heartbeat periods from the acquired projection data, each of the first and second projection data sets covering an angle range required for the reconstruction of one frame image, a processing unit weighting each of the extracted first and second projection data sets with a weight corresponding to a data acquisition position and generating a third data set by combining the weighted first and second projection data sets, and a reconstruction processing unit reconstructing one frame image data set on the basis of the generated third data set.