An X-ray detector is provided with a plurality of modules each having a plurality of detection elements each composing a pixel, in which the detection elements convert incoming radiations to electric data depending on amounts of the radiations. The plural modules are mutually adjacently arranged on the same surface with a gap having a known width formed therebetween, such that the modules are arranged along at least one of a first X-axis and a first Y-axis, wherein the radiation detector is given a scan direction which is set along one of the first X- and Y-axes and the first Y-axis is perpendicular to the first X-axis. The plural detection elements of each module are two-dimensionally arranged along a second X-axis and a second Y-axis which are set obliquely to the first X-axis and the first Y-axis respectively and which are perpendicular to each other.
