An X-ray CT apparatus includes an X-ray irradiation unit for applying X-rays based on a first X-ray tube voltage and X-rays based on a second X-ray tube voltage to a subject by being switched every one view, a projection data acquisition unit for acquiring projection data by which X-ray tube voltage information about the applied X-rays are identified, and an image reconstruction unit for identifying first energy projection data based on the first X-ray tube voltage, second energy projection data based on the second X-ray tube voltage, and transient energy projection data acquired upon switching between the first and second X-ray tube voltages based on the X-ray tube voltage information, the image reconstruction unit including a conversion processor that converts the transient energy projection data to another data using the transient energy projection data and performs image reconstruction using the data subsequent to the conversion process.
