To provide a radiation tomographic image generating apparatus capable of easily acquiring a radiation tomographic image on which the image estimation of a truncation portion is performed almost without the need of the optimization of a parameter, and to provide a radiation tomographic apparatus and a radiation tomographic image generating program.

An X-ray CT image creating section 51 successively performs approximate image reconstruction using merely measured projection data in a reconstructed image region that includes a circular region set larger than a circular region whose diameter is the data width of the measured projection data. An image of a truncation portion can be estimated only with the complete measured projection data even when the measured projection data is truncated by setting the reconstructed image region to be large. The complete measured projection data have no error caused by having an estimated portion in projection data for generating a radiation tomographic image. Artifacts are thereby reduced with high matching properties between projection data.