An X-ray CT apparatus according to an embodiment includes processing circuitry. The processing circuitry is configured to obtain initial temperature information of a photon counting detector before a main scan, information about the shape of a subject, and a scan condition of the main scan. The processing circuitry is configured to estimate a temperature change of the photon counting detector to be observed when the main scan is performed, on the basis of the initial temperature information, the information about the shape of the subject, and information about the scan condition of the main scan. The processing circuitry is configured to judge whether or not it is possible to perform the main scan, on the basis of the temperature change and the initial temperature information.