1. A method of producing a cardiac tomogram using an X-ray CT apparatus performing helical scans which has multiple columns of X-ray detector in a slicing direction of an object to be examined and using projection data detected by the X-ray detector, comprising the steps of: measuring electrocardiographic information of the object; calculating a number of projection data to be divided necessary for reconstruction of a tomogram in each of the helical scan; collecting divided projection data in an identical heartbeat time phase on the basis of the number of projection data to be divided and an electrocardiographic cycle obtained in the step of measuring the electrocardiographic information, and forming projection data necessary for reconstruction of a tomogram; interpolating a discontinuous portion of the above formed projection data in the slicing direction of the object and forming continuous projection data; and forming a cardiac tomogram of the object using the continuous projection data.