1. A radiation imaging apparatus, comprising: a radiation emitting source that emits radiation; a radiation detector that outputs, every frame, electric digital two-dimensional data corresponding to incidence of the radiation to the radiation detector; moving means for moving a pair of the radiation emitting source and the radiation detector, the radiation detector, or an object being imaged relatively to a remaining one among the radiation emitting source, the radiation detector, and the object; data acquiring means for acquiring, every frame, the data outputted from the radiation detector while the pair of the radiation emitting source and the radiation detector, the radiation detector, or the object is moved relatively to the remaining one by the moving means; image producing means for producing a plurality of three-dimensional optimally focused images on the basis of the data acquired by the data acquiring means at different time points from the same portion to be imaged of the object, the portion to be imaged of the object being optimally focused and reflecting an actual position and shape of the portion therein; and estimating means for estimating an amount of changes among the plurality of three-dimensional optimally focused images produced by the image producing means at the different time points.