An X-ray CT (computed tomography) apparatus is disclosed. The X-ray CT apparatus include an X-ray supply unit performing low-dose X-ray scanning and high-dose X-ray scanning for a patient in which a contrast medium has been injected. The collected X-ray projection data are reconstructed to generate image data for low-dose X-ray scanning and high-dose X-ray scanning. In the low-dose X-ray scanning that proceed the high-dose X-ray scanning, plural images are obtained in succession, in which ROIs (regions of interest) are set in given positions. A CT value calculating unit calculates the CT values in the ROIs in succession, and the timing at which the high-dose X-ray scanning is started is determined on the basis of the changes of CT values with time. And blood-flow information is calculated by using the image data obtained with low-dose X-ray scanning and high-dose X-ray scanning.