There is provided an X-ray CT apparatus capable of correcting the unevenness of the contrast concentration due to discontinuities in time between slices in contrast imaging using a Prospective Triggering method. In an X-ray CT apparatus 1, projection data in a specific phase of the heart is obtained by performing a scan by emitting X-rays after a predetermined time from the detection of the R wave of electrocardiogram information at each position of the heart in the body axis direction. An image processing device 4 reconstructs a tomographic image on the basis of collected projection data, calculates the projection data collection time in each scan from electrocardiogram information collected from an electrocardiograph 5, calculates the time between scans on the basis of the calculated projection data collection time, and corrects a concentration difference so as to change smoothly for an image of a position in the body axis direction in a range according to the time between scans.