An X-ray CT apparatus of an embodiment includes a processing circuitry. The processing circuitry detects X-rays radiated from an X-ray tube in units of photons. The processing circuitry stores first energy spectrum information acquired by detecting X-rays at a first timing. The processing circuitry acquires second energy spectrum information by detecting X-rays at a second timing after the first timing. The processing circuitry determines a state of the X-ray tube on the basis of the first energy spectrum information and the second energy spectrum information.