X-ray CT apparatus is provided in which the photon energy distribution of X-rays to be radiated is flattened. X-ray CT apparatus includes an X-ray tube, a detector, a data acquisition system, a tube voltage generator, and a grid controller. The X-ray tube radiates X-rays onto a subject. The detector includes multiple detection elements for detecting photons forming the X-rays. The data acquisition system counts the number of the detected photons to acquire projection data based on the counted photons. The tube voltage generator applies the tube voltage to the X-ray tube while changing the tube voltage of the X-ray tube in a predetermined cycle. A tube current controller decreases the tube current upon an increase in the tube voltage, and increases the tube current upon a decrease in the tube voltage. Thus, the photon energy distribution of the X-rays radiated from the X-ray tube is flattened.
