Field infiltration tests were conducted to study the effects of understories on sustaining high permeability in forested areas. Ten field plots were set up in Chamaecyparis obtusa (S. et Z.) ENDL. plantations, in which five plots had understories, and the remainder had no understories. The 1m × 1m rain simulator tests showed that the formation of the surface crust is responsible for the infiltration decrease rather than the pore size distribution of the surface soil. Soil crusts were observed on bare soils, which suggested that understories and litter layers prevent the forming of surface crusts and sustain high infiltration rates.