The differences in the water permeability and water retention capacity were examined between the adjacent coniferous forest (Chamaecyparis obtusa) and grassland (Miscanthus sinensis) soils (Andosol) on the somma of Mt. Aso, Japan. The saturated hydraulic conductivity of forest soils (27.6 mm h－1) was 4.4―fold higher than that of grassland soils (6.3mm h－1). Although total porosity did not differ between the vegetation soil types (approximately 80 %), non‐capillary and coarse capillary porosities in the forest soils (7.2 % and 13.3 %, respectively) were significantly larger than those in the grassland soils (4.5 % and 9.2 %, respectively). These results suggest that forests aid disaster prevention and water source protection by improving the water permeability and water retention capacity of soil in this area.