A soil-shielding benchmark experiment was conducted using secondary particles produced by 12 GeV protons, which were injected into an iron rod surrounded by soil. Induced activities of Na-22 in aluminium (Al) and soil samples were measured and the experiment was simulated by the MARS Monte Carlo code. The induced activities in Al samples were calculated using spallation cross sections and fluence, where the fluence was calculated by the MARS code. The relation between flux density and induced activities in soil was investigated using calculated flux densities: the distribution of the ratio of induced activities in soil samples to the flux densities showed the radial and axial independence. Both saturated activities and distribution coincide with experimental data within the estimated errors. These results indicate the successful simulation by the MARS code.