Usually, radon diffusion coefficients in soils are measured based on the assumption that Fick's law holds true. But, unlike in a free volume, strictly speaking, there is no guarantee that Fick's law holds in soils. Transient-diffusion measurements of radon in soils are best employed in order to confirm Fick's law. Alpha-particles from Po-218/Po-214 and from Rn-222 cannot be distinguished in the course of those measurements. Therefore, the relevant portions of the detector reading must be allocated to each of these nuclides. For that purpose, the behaviors of Po-218 and Po-214 in the detection volume were determined and their ratios of contribution to the detector reading were also determined by Monte Carlo calculations and a verification test. This information was used to confirm Fick's law for representative Japanese sand and loam samples through indicial transient-diffusion measurements and a theoretical interpretation of the measured data.