An X-ray detector for use in a medical X-ray imaging apparatus comprising a rotary means rotatable relative to an object to be examined, an X-ray generator provided at one side of the rotary means, and an X-ray detecting portion provided at the other side of the rotary means so as to face the X-ray generator. The X-ray detector is provided in the X-ray detecting portion or is detachably mounted in the X-ray detecting portion, the X-ray detector is provided with an imaging portion comprised of a plane electric imaging means extending in a two-dimensional direction used for X-ray CT and has an imaging portion positioning means for moving up and down the imaging portion in the X-ray detector.
