To provide an X-ray detector facilitating the installing and replacement work of a module while reducing the possibility of breakage. An X-ray detector 50 detecting X-ray image data for each detection module includes: a detection module 7 provided with a protruding frame on a back side of a detection device detecting X-rays; and a guide frame 12 fitting into the protruding frame and removably supporting the detection device, wherein the guide frame 12 fixes the position of the detection device relative to the guide frame 12 by fitting. Therefore, fitting the protruding frame 8 into the guide frame 12 enables precise and easy installation/removal of the detection module. That is a detection module can be newly installed onto the guide frame without interfering each other with adjacent detection modules already installed while minimizing a space therebetween.