1. A radiation imaging apparatus comprising: a radiation source that emits a radiation; a radiation detector that outputs, frame by frame, digital-quantity two-dimensional data corresponding to incidence of the radiation thereto; moving means for moving a pair of the radiation source and the radiation detector such that the radiation is scanned to focus on a desired referential tomographic plane of the object, the radiation source and the radiation detector being arranged to be opposed to each other with the object located therebetween; data acquiring means for acquiring, frame by frame, the digital-quantity two-dimensional data outputted from the radiation detector while the pair of the radiation source and the radiation detector is moved relative to the object; and image producing means for producing a three-dimensional optimally focused image based on the data acquired by the data acquiring means, a portion to be imaged of the object in the three-dimensional optically focused image being optimally focused and reflecting an actual size and shape of the portion thereon; wherein the image producing means comprises: referential-plane image reconstructing means for reconstructing, as a referential-plane image, a projection image based on the data acquired by the data acquiring means, the projection image being produced by projecting the referential tomographic plane onto a detection surface of the radiation detector, and optimally-focused image producing means for producing the three-dimensional optimally focused image based on data of the referential-plane image and the frame by frame data.