An X-ray CT apparatus comprising an X-ray source for irradiating a subject with X-rays, an X-ray detector disposed oppositely to the X-ray source with the subject therebetween and detecting transmission X-rays of the subject as projection data, a means for rotating the X-ray source and the X-ray detector, a control means for collecting the projection data in a plurality angular directions when anX-ray tube and the X-ray detector are rotated by the rotating means and computing the projection data thus collected for reconstruction to create a CT image of the subject and controlling the X-ray source and the rotating means, and a means for displaying the created CT image. The X-ray CT system further comprises a projection data analysis means for reconstructing the CT image for analysis at the imaged portion of the subject from the projection data and reprojecting the reconstructed CT image to generate a control profile, and a tube current control means for controlling the value of the current supplied to the X-ray tube according to the control profile thus created.