A direct method for numerical investigation of radiation trapping within the framework of the Holstein-Biberman equation is put forward. The method is based on time propagation of the initial distribution of excited atoms with the split-propagation technique. Some results obtained for circular and elliptic geometries of the gas cells are given in comparison with the data of previous calculations. The present method allows one to investigate various transfer phenomena occurring in an arbitrary (three-dimensional) convex spatial region and driven by the external conditions with arbitrary time dependences.