Provided is a radiation detector including a scintillator which generates, when a radial ray enters, scintillation light having light intensity according to energy of the radial ray, and then supplies a photon of the scintillation light to each of a plurality of pixels, a radial ray detection unit which detects whether or not the radial ray is made to enter based on a number of the photons supplied in an exposure period whenever the plurality of pixels are exposed by the scintillation light over the exposure period, and an exposure period adjusting unit which adjusts the exposure period based on an incident frequency of the detected radial ray.
