Abstract: A holding device for an X-ray plate adapted for mounting on an X-ray tube comprising a ring mountable on the X-ray tube for turning movement thereon. The ring is supported in a groove formed by two auxiliary rings which can lock the ring in a fixed angular position on the tube. A bar is attached by a detachable fastener assembly attached to the ring in a position in which the bar extends parallel to the axis of the tube. The bar is rotatable around its own axis and is longitudinally displaceable in the fastener assembly. The bar extends forwardly from the fastener assembly and has a front end at which there is detachably supported a throw-away X-ray plate-holder extending at right angles to the bar. An X-ray plate is mounted in a slit in the plate-holder and assumes a position at right angles to the axis of the tube.