With a view to providing an X-ray irradiator capable of detecting a relative rotational angle of a collimator box with respect to an X-ray tube, the X-ray irradiator comprises: an X-ray tube having a flange formed around an X-ray output window; a collimator box having an entry plate formed with an X-ray incidence window, the collimator box accommodating a collimator in the interior thereof; a ring provided on the entry plate so as to surround the X-ray incidence window, the ring receiving therein the flange of the X-ray tube and having a plurality of slots formed radially through the ring; a plurality of tongue pieces being inserted through the slots from outside to inside of the ring and holding down a back side of the flange; and sensor device provided inside the flange to detect a relative rotational angle of the collimator box with respect to the flange.