An X-ray CT apparatus is provided that generates a three-dimensional cross-section model from scanogram projection data of an examinee. An operator inputs a site of interest, an assumed displacement amount of the site and a desired image quality index value. A scan planning unit calculates an X-ray attenuation index from the three-dimensional cross-section model, and corrects the calculated X-ray attenuation index on the basis of the input site of interest, assumed displacement amount and image quality index value. The scan planning unit further determines a tube current modulating pattern (irradiation X-ray dose modulating pattern) on the basis of the corrected X-ray attenuation index. When scanning is executed, an X-ray tube is controlled according to the determined irradiation X-ray dose modulating pattern, and controlled so as to obtain an optimum X-ray dose.