An approach was proposed to compute radiation dose to objects of interest in infinitely expanded radiation fields with Monte Carlo transport codes. The radiations, which were emitted far from the interested object, could be effectively taken into account by setting reflection boundaries surrounding the computational volume in this approach. Here, the positions and momenta of the radiations were recorded at each reflection and used as the sources to simulate radiations from the exterior region of the computational volume. The validity of this approach was checked by radiation transport calculations of objects on the infinitely expanded contaminated ground surface. The results in this approach agreed within statistical errors of the simulation with those in the conventional approach considering a very large computational volume. The required computational time of the authors' approach for a desired statistical uncertainty was a hundred times shorter than that of the conventional approach.