In an X-ray CT apparatus of an embodiment, an X-ray tube emits X-rays. A detector detects the X-rays emitted from the X-ray tube and having passed through a subject. Processing circuitry collects projection data based on data detected by the detector. The Processing circuitry generates a reconstructed image from the projection data. A display displays a display image based on the reconstructed image. A input circuitry receives an operation to rotate a first display image based on a first reconstructed image generated by the processing circuitry on a screen of the display, and specify a certain region on a second display image whose axis is in a direction different from a slice direction. The processing circuitry generates a second reconstructed image from the projection data, having higher resolution than that of the first display image, for the certain region.