An X-ray detector and an X-ray CT apparatus that facilitate collimator plate arrangement are characterized by comprising radiation detection element arrays in which a plurality of radiation detection elements detecting a radiation generated from a radiation source are arranged in a first direction and a second direction orthogonal to the first direction, collimator plates that are arranged along the first direction on the radiation source side of the radiation detection element arrays to remove scattered radiations, and collimator plate support members that have grooves supporting the collimator plate and are arranged along the second direction between the radiation detection elements.
