PROBLEM TO BE SOLVED: To provide an X-ray CT apparatus capable of optimizing its tube voltage and tube current.SOLUTION: Based on scout data acquired by scout photography and a tube voltage V1 of an X-ray tube, a tube current I1 of the X-ray tube is calculated so that an image of a required picture quality level SD can be acquired when a subject is scanned with the tube voltage V1 combined (S103). Then a tube voltage V2 (