Abstract: A method and an apparatus for obtaining radioactivity, radioactivity concentration and radioactivity surface density without using an actual calibration radiation source but utilizing a &ggr; ray, including: a virtual modeling step (21) for arranging virtual three-dimensional models of a measurement subject and a radiation detector in a virtual three-dimensional space with the same positional relationship as an actual geometric positional relationship; a conversion factor setting step which is a step for associating the number of times of generation of a virtual radiation ray emitted from a virtual three-dimensional model of the measurement subject with the number of times of incidence of the virtual radiation ray upon a virtual three-dimensional model of the radiation detector to obtain a conversion factor, and has a virtual count calculating step (22) and a conversion factor calculating step (23); an actual counting rate calculating step (24) for actually counting the incidence of the radiation ray emitted