A radiation dose modulation apparatus controls radiation dose in coronary CT angiography that utilizes an X-ray CT system including a multi-row detector. The apparatus is provided with a tube current modulation table storage unit which classifies standard deviation values of CT numbers calculated from a pre-contrast cardiac CT image into a number of levels, and stores a table in which the levels correspond to tube current modulation values, and a processing unit which calculates standard deviations of CT numbers from at least one pre-contrast cardiac image, identifies the tube current value corresponding to the standard deviations from the tube current modulation table, and controls a scan operation at the time of CT imaging for obtaining a post-contrast coronary image in accordance with the identified tube current value.