This invention is provided to determine a tube current at the imaging of the heart by an X-ray CT apparatus in such a manner that image noise becomes constant. Upon determining a tube current of an X-ray tube at the time that the heart is imaged by an X-ray CT apparatus, the tube current is determined based on an index related to image's noise and a BMI of a subject. The noise is at a central portion of an ascending main artery. Determining the tube current is conducted using a function with the BNI as a variable. The function is given in the form of a polynomial equation of the BMI. Each coefficient of the polynomial equation is a function of the index.