The present invention provides, at low cost, a multilayer radiation detector whose position relative to a beam axis can be verified. The radiation detector includes a plurality of sensors that react to radiation and are stacked in parallel inlayers in a traveling direction of the radiation. The sensors are each sectioned into a central region including the center of the sensor and another region surrounding the central region. The radiation detector independently measures signals measured by the central regions and signals measured by the other regions. Thus, the position of the radiation detector can be verified.