A radiation imaging apparatus detects the density distribution of an image formed by a radiopaque dye from each of two images obtained by radiation imaging. The apparatus predicts the moving speed of the image formed by the radiopaque dye based on the moving amount of the detected density distribution and the interval between the radiographing times of the two images, and determines the next timing of radiation imaging based on the predicted moving speed and the detected density distribution. A radiation imaging apparatus detects the density distribution of a radiopaque dye in an image obtained by X-ray radiation imaging and predicts the moving speed of the radiopaque dye based on the change amount of the density distribution. The radiation imaging apparatus determines the X-ray irradiation timing based on the predicted radiopaque dye moving speed, the detected radiopaque dye density distribution, and a predetermined radiopaque dye density.