Previously, the author guessed as follows, in accordance with the statistical study on landslides of the Tenryu river caused by heavy rain of June 1961, that is: the increase of total area of landslides in a second order drainage basin is performed by the increase of number of landslides but not by the expansion of area of each landslides.
Using same data, the following results were obtained according to the subsequent study.
(1) Number of landslide per unit area of drainage basin is affected by amount of a downpour and geology of the basin.
(2) On the contrary, mean area of a landslide is not affected by these two factors, but intensively affected by the drainage density.
(3) The fact that the difference of affecting factors between number and mean area of landslides indicates the difficulty of getting the accurate knowledge about the landslides with using the hazy dimension such as area of landslides per unit area of a drainage basin, which is the product of number and mean area of landslide.
(4) A tendency that the drainage basin of having many landslides in a ordinal condition generates many landslides by downpour, was found. And forest condition has no effects on the breaking out of the landslides.
(5) On the Shitoku river basin, a tributary of the Tenryu river, the mountain slope of having gradient of from 50 to 55 drgree are most easy to generates the landslides. From the dynamic consideration steep slope is ought to slides down easily, but the foundation to be able to sufficiently supports the following presumption was found out, that is steep slope is hard to generates the landslides as it has not enough surface soil to slides down.
(6) Within the slope of having gradient less than 50 or 45 degree, the number of landslides taking place on a given slope gradient per unit basin area is represents the Poisson distribution. And the parameter (λ) of the distribution increases with increase of slope gradient. This fact coincides with the theoretical consideration, and it makes the phenomenon of landslide occurance within a drainage basin to be able to be treated as a problem of probability.
Of course, these results have been obtained only on the Tenryu river basin, and it is not sure whether these relations are recognized in general river basin or not.

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