Abstract
　Dependence of surface velocity of P-wave on the landform parameters, the equivalent slope length I and the slope gradient I, defines the obliquity of iso-velocity surface against land surface. The angular relationship of both surfaces is resulted from the essential nature of weathering described by diffusion equation with surface evaporation boundary condition and from simultaneous erosion of uppermost portion of weathered zone. The average duration time of weathering at a given point of mountain slope is obtained by comparison of actual obliqueness with that on model slope. The duration time is over 100 years near ridge part and 13-30 years approximately at the point 5m apart from the ridge. The former value means that the surface material distributed there has suffered weathering for a long time and is staying keeping pace with frequent but weak erosive force corresponding to small value of l and I. The latter value on the contrary means deep-seated denudation by slope failure and is identical with the interval required for recovery of weathered mantle after occurrence of the failure.

【キーワード】：風化，斜面，拡散，平均風化時間
Key words: weathering, slope diffusion, average weathering period.

*大阪市立大学文学部地理学教室
Department of Geography, Fac. Lit., Osaka City Univ.