[en] Radon (²²²Rn) diffusion coefficient as one of some important parameters of ²²²Rn transport in the soil has been measured using soil radon diffusion monitor (SRDM). The system of SRDM consists of two scintillation detectors, sample container located between these detectors, photomultiplier tubes, amplifiers, single channel analyzers and personal computer. The source of ²²²Rn is put in the one of the compartment, and ²²²Rn will diffuse through the sample to other compartment. The diffusion coefficient was determined by calculating the measured data from two detectors using time-dependent diffusion model. The samples come from different places in Japan with several porosities and water contents. The diffusion coefficients were found to vary from (8.86±0.23) x 10^-7 m²s^-1 to (4.39±0.43) x 10^-6 m²s^-1. Furthermore, the correlations between ²²²Rn diffusion and water content and/or porosity are also discussed. (author)