In order to develop a prediciion method for the occurring time of a slope failure based on the critical rainfall, the following studies were carried out. Conditions which has influence on the critical rainfall were discussed. Then, the change of pressure head in natural condition were investigated. Finally, the pore pressure distribution in a slope and the stability of slope were analysed numerically.
Consequently, the following results are obtained:
(1) The critical rainfall curve which is the relation between rainfall per a week and rainfall per a day of the failed slopes can be obtained numerically.
(2) The seepage characteristic and the thickness of permeable layer have a great influence on the total rainfall till the slope failure.
(3) The quantity of successive rainfall for the slope failure can be accurately estimated by numerical analysis if the initial pore pressure distribution in the slope is known.

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The author studied photographic interpretation of landslide areas and collapsed places in some regions of Japan, and grasped geomorphological features in the studied areas by some geomorphological methods, and considered with the viewpoint of historical development of landforms by comparison with geological structures. In this paper geomorphological features and changes of some large landslides is explained by photographic interpretation, and discussed the necessity of studies of historical development of landforms for scientific researches of landslide mechanism.

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Many landslides are found in area of tertiary rocks sliding mass moved along thin clay layer on bed rock in Amakusa island.
The authers explain geological mechanism of landslide by comparing accumulative strain and accumula tive amount of precipitation, and found that geological environment (specially capacity of ground water) has effected on landslide behaviour.

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Ground water drainage works by horizontal bore holes occupies very important position in landslide preventive works. And a large sum of expense is required for carrying out this works. Nevertheless, it is very grave problem that ground water does not drain from some horizontal drainage bore holes in the rainy season.
Temperature logging was carried out in some horizontal drainage bore holes as an effective means for investigating the cause of this problem. This logging is recognized as very effective means for detecting flowing ground water strata in vertical bore holes.
As a result of these investigation, next matter are cleared:
1) By means of comparing with normal ground temperature curve taking account of thickness of soil layer and obtained temperature curve in situ, we can obtain some significant information on places of flowing and leaking positions of ground water in horizontal drainage bore hole.
2) Furthermore, we can obtain useful data which are able to discuss that the horizontal drainage bore hole carried out temperature logging is exist in ground water vein stream or not.
The last of in this paper, we propose some improvable measures on the basis of the result of investigation in this time.

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The paper reports on the effectiveness of the natural radioactivity survey and the electrical prospecting. According to the result of the investigation which has been carried out scince 1963, the following became clear.
1) Relation between the groundwater zone and natural radioactivity values, apparent resistivity values.(Table 1)
2) Relation between the boundary of lithology and natural radioactivity values. Many landslides have occurred on the boundary part of different rocks and these boundary parts can be estimated from the natural radioactivity survey.
3) Relation between the boundary of lithology and apparent resistivity values.
As Mikabu greenrocks zone is rich in clay and the groundwater, apparent resistivity values are relatively lower than those in Sanbagawa zone (north side on Mikabu zone) and Chichibu zone (south side on Mikabu zone).
In the part of pillow lava and breccia apparent resistivity values often come to extremely low in the same Mikabu greenrocks zone.

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