There are provided an X-ray detector, which can reduce the deformation of a collimator plate and can be easily processed and installed, and an X-ray CT apparatus using it. The X-ray detector includes a collimator plate, and a scintillator array, a photoelectric conversion element array and a substrate that are bonded in order from the X-ray incidence direction. The collimator plate is disposed such that one of a pair of opposite sides of the collimator plate is bonded to a lower support plate bonded on the scintillator array and the other side is bonded to an upper support plate and directions of the opposite sides are the same as a rotation axis direction of an X-ray CT apparatus in which the X-ray detector is provided.