1. An X-ray CT apparatus, comprising: an X-ray generator that emits X-rays from periphery of an object; an X-ray detector that detects X-rays transmitted through the object; a data acquisition system that acquires data detected by the X-ray detector; a reconstruction arithmetic device that creates projection data by receiving the data acquired by the data acquisition system and reconstructs a CT image using the projection data; and a display device that displays the CT image, wherein the reconstruction arithmetic device includes: a number-of-iterations setting section that sets the number of iterations of iterative processing for correcting the projection data; a calibration coefficient calculating section that calculates a calibration coefficient for adjusting an application ratio of a first processing function and a second processing function having a different characteristic from the first processing function, the first and second processing functions being included in the iterative processing; a successive approximation projection data correction processing section that creates corrected projection data by performing the iterative processing on the projection data based on the number of iterations and the calibration coefficient; and an image reconstruction section that reconstructs the CT image using the corrected projection data.