1. A radiation imaging apparatus that irradiates a subject with a radioactive ray emitted from a radiation generation unit to capture a radiographic image based on a radioactive ray that has penetrated through the subject, the radiation imaging apparatus comprising: a histogram calculation unit configured to calculate a histogram representing a frequency of each pixel value resulting from pixel values of the radiographic image; and a radiation irradiation condition determination unit configured to determine radiation irradiation conditions for a radioactive ray to be emitted from the radiation generation unit in such a way as to flatten a distribution of the frequency of the histogram.