1. An X-ray CT apparatus, comprising: an X-ray irradiation unit that irradiates X-rays from periphery of an object; an X-ray detection unit that detects information of X-rays transmitted through the object; an electrocardiographic information acquisition unit that acquires electrocardiographic information of the object; an image creation unit that creates an image of the object from the X-ray information and the electrocardiographic information; a display unit that displays the image; a tomographic data creation unit that creates a plurality of pieces of tomographic data with different cardiac phases from the X-ray information and the electrocardiographic information; a region data generation unit that generates region data by extracting a predetermined region for each piece of the tomographic data; a variation distribution calculation unit that calculates a variation distribution within the tomographic data by calculating a variation between cardiac phases regarding the region data; and a degree-of-harmony calculation unit that calculates a degree of harmony, which is an index indicating whether or not rhythms at respective locations of a target portion of the object are in harmony with each other, on the basis of the variation distribution.