In one embodiment, an X-ray CT apparatus includes an X-ray tube that radiates, an X-ray, a detector that outputs a plurality of pieces of pre-compression data on a basis of the X-ray first processing circuitry and second processing circuitry. The first processing circuitry groups the plurality of pieces of pre-compression data to generate grouped data corresponding to the pre-compression data in each group, and generates data for restoration for restoring the pre-compression data, wherein the first processing circuitry transmits the grouped data and the data for restoration to the second processing circuitry. And the second processing circuitry selects any of a first reconstruction mode in which image reconstruction is performed based on the plurality of pieces of pre-compression data; and a second reconstruction mode in which image reconstruction is performed based on the data for restoration, and generates an image, and in a case of performing image reconstruction in the first reconstruction mode, the second processing circuitry restores the pre-compression data based on the grouped data and the data for restoration.