[Problems] To provide an X-ray CT apparatus capable of easily comparing image quality supposed to be obtained when not using a X-ray dose optimization function and image quality supposed to be obtained when using the X-ray dose optimization function at the stage of scan planning.[Means for Solving Problems] A 3-dimensional model of an object to be examined (17) is generated from scanogram projection data (S170). An image noise dispersion value corresponding to an imaging region of the object is predicted from the 3-dimensional model.The predicted image noise dispersion value is compared to a desired value of the image index value inputted by a user so as to calculate a modulation pattern of the irradiation X-ray amount (S200).By predicting the image quality supposed to be obtained when using and not using the X-ray dose optimization function, each of the predicted results is displayed so as to be compared on a display device (5) (S230).