The present paper reports that the most important problem in prediction of failure hazards is how to select the hazardoues slopes out of numerous slopes; therefore it is necessary to take the suitable prediction methods according to each characteristics of slopes which are classified into several mode. Moreover the problems and limit of general statistical methods for ranking of hazardous level, and the standard of classification of hazardous level are stated.

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The writer picks up marked heavy storms which caused many slope slides at miscellaneous places in our country and moreover the time when a comparatively large-scale slides happened is distinct. And then, he described rainfall mass curves and ploted an arrow or arrows indicating the time of slides on each curve. The relations among slope characteristics, accumulated rainfall by the said time, and steep-slope portion of the curve at each slope, are carefully examined by the writer. And also, whether the said rainfall which infiltrated into the slope surface layer was able to build up a certain stored water depth on the less permeable subsurface layer which is one of the necessary conditions for surface slides at each slope, was carefully investigated. The writer concluded that there are frequent occaisions when only the infiltrated water can not cause the slide and at the same time ground water in fractured zones existing at the slope, increases its water head and discharge from the fracture tops which forms the stored water in the surface layer in the case of violent rain.

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Regarding the relation of forest conditions and occurrences of landslides having relatively shallow depth on steep slopes, there are some oppinions that the existence of forest may have a reverse effect in landslide prevention.
After investigating several characteristics of forest lands and states of landslide occurrences on many mountaineous areas, I induce that good forests would generally display the good function of slope failure prevention. This is mainly owing to the role of forest root systems which fasten the soil so tight that the strength of weathered surface soil layers increases tolerably.
On the prevention of individual small failures, however, there are many unsolved problems such as figures of root systems on various soil conditions or the meaning of them in soil mechanics.

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A method of predicting, in an indirect manner, a big likelihood of landslide resulting from a prolonged rainfall or a cloudburst, on the basis of analysis of the information on the rainfall of present and during a plurality of past days for declaring timely warning for safety-evacuation of the local inhabitants is indicated, together with the conception and description of setup of unique Effective Rain Gauge ingeniously worked out by the author, with a special concern given to the facilitation of the declaration of warning.

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The Geographical Survey Institute made an investigation of the landslides in Amakusakamishima Island caused by the heavy rainfall of July, 1972, mainly from geomorphological viewpoints and made a quantitative estimation of danger of landslides by a statistical method. The report is summarized as follows.
1. The heavy rainfall caused about 4, 600 small debris avalanches and very few large slumps of bedrock on mountain slopes of the eastern part of the island. In the aerial photographs taken in 1965, 240 scars are observed and 70 of those were scarred again due to this rainfall.
2. These debris avalanches originated a large number of debris flows. Where a hard arkose sandstone of Palaeogene or volcanic rocks are present, debris flows became large and destructive owing to the very large blocks of these rocks.
3. Most of the debris avalanches and flows occurred when the rainfall was extremely intensive, between 11 and 13 o'clock of July 6, while the large slumps seems to have occurred after the rain.
4. The relation between distribution of debris avalanches and the age of mountainslope was studied. Knick-lines on which the slope gradient changes discontinuously, from the upper gentle to the lower steep, were read in a stereo-airphotographs and were used as erosion fronts for classifying the slope age. An erosion front indicates the borderline of an erosion area which spreads itself with time. The age of those in the south of Amakusa is divided into four.
1st, originated in the middle and late Pleistocene.
2nd; originated in the late Pleistocene.
3rd; originated during the regression of the late Pleistocene and early Recent.
4th; originated during the Recent transgression.
Debris avalanches intensively occurred in the 2nd and 3rd erosion areas, while not so many occurred in the 4th. It is probable that the 4th area had not had much detritus to move down due to its short periodicity of erosional activities.
5. The smaller limit of a watershed above a scar and its average gradient were found to be 40-50m² and around 1/4 by measuring 1/2, 000 topographic maps.
6. An equation for estimating the magnitude of debris avalanche was built by means of multivariate analysis in the area A where the precipitation was very high. Six factors were adopted as the predictor variables, i.e. rock, slope, drainage density, thickness of detritus, vegetation, and orientation of slope. The equation was applied to an estimation for another area B in the island, where the precipitation was much less than in A. The estimated magnitude is much larger than the real one.

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Natural slope Failure deals with many complicated factors, such as rainfall pattern, properties of soil, ground water, and seepage water.
At present, we do not have enough technique for treatment of natural slope stability. So, it is useful that we study the behaviour of natural slope failure by field observation data.
We pile up data from 201 spots where natural slope failure actually occurred from 1969 to 1971, and examine their characteristics, by extracting 18 elements, such as failure pattern, height and width of failure, and amount of rainfall. As for the failure pattern, slide of surface sail has eminent position and its avarage thickness is 2m. As for the rain fall, it is reasonable, that it reaches critical point for danger of slope failure, when contineous rainfall amounts 100mm, or 20mm for 24 hours.

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[in Japanese]

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