1. An X-ray image processing apparatus comprising: a calculating unit adapted to calculate a noise amount of an X-ray receiving unit on the basis of a difference value of a plurality of dark images acquired at different timings by said X-ray receiving unit when no X-rays are irradiated; a changing unit adapted to change a predetermined parameter for processing an X-ray image acquired by said X-ray receiving unit when X-rays are irradiated, in order to prevent the noise amount from being superposed on the X-ray image; and an image processing unit adapted to perform image processing on the X-ray image on the basis of the parameter changed by said changing unit, wherein said calculating unit calculates the noise amount on the basis of a difference value between a first dark image acquired by said X-ray receiving unit before start of X-ray irradiation, and a second dark image acquired by said X-ray receiving unit after end of X-ray irradiation, and said calculating unit calculates the noise amount on the basis of a difference value between the first dark image, and a third dark image acquired by said X-ray receiving unit when a predetermined time has elapsed after acquisition of the second dark image.