Abstract
　Landslides occur as a result of combination of many causative factors such as topography, geology, vegetation, and the like. Of these factors, it is the biological one that we are able to control to prevent the outbreak of landslides.
　In order to investigate the possible relationship between the occurrence of landslides and the age of the forest stand covering the ground, we made a comparison in porosity and water infiltration in surface soils between a mature stand and a neighboring juvenile one artificially planted after clearcutting of preceding old one, and obtained the following results.
l）Infiltrated water through the surface soils in juvenile stand reached deeper layer more rapidly and abundantly than in mature one.
2）Standard deviation of porosity in juvenile stand was larger than in mature one.
3）It was considered that the difference in porosity was due to qualitative in surface soil through changes in micro-meteorology and interruption in the supply of fresh organic matters caused by clearcutting of the preceding old stand.

*名古屋大学農学部
Faculty of Agriculture, Nagoya University