A radiation detector according to an embodiment includes a plurality of radiation detector modules and a fixing frame. When a radiation detector module is attached to the fixing frame, a guiding part with a groove shape formed at an end of a support member of the radiation detector module on a side of the fixing frame is fitted to a guiding pin provided to an end of the fixing frame, so that the radiation detector module is positioned in a radiation irradiation direction while a movement thereof in a channel direction and a row direction is restricted.
