An X-ray image diagnostic apparatus according to an embodiment includes a photon counting X-ray detector, calibration circuitry, and image generating circuitry. The photon counting X-ray detector includes a plurality of detection elements each of which is configured to detect X-rays and output a detection signal. The calibration circuitry calibrates a detection signal of each of the detection elements using a correction value calculated from a plurality of signals output from the detection elements and corresponding to a plurality of X-ray application conditions relating to continuous X-rays. The image generating circuitry generates an image using the calibrated detection signals of the detection elements.