The safety factor of 2-dimensional analysis used by countermeasure plan of a landslide serves as a value which included the error with various indefinite elements about the calculation error and stability analysis by the stability analysis formula to adopt. Although the error of asfety factor has influenced on effect presumption of countermeasure greatly, there is almost no verification example with the general-purpose but only with individual on-site example. On the other hand, though the designed safety factor in countermeasure plan is an important value which determines the scale of countermeasure, the validity of the value owes a lot to experience, and it is not verified scientifically yet. By verifying the influence of the indefinite element on effect of countermeasure generally, I think in the future, it will be also useful to verification of the validity of designed safety factor. In this paper, I took notice of 3 main indefinite elements, the unit weight of move layer, strength parameters of slip surface, slip surface depth. Then verified influence of these 3 elements on effect of couner measurements, groundwater drainage works, soil removal works, prevention works by using the 133 spot and 243 section of on-site data all over the country. I also verified about the superposition effect of the presumed error by two or more indefinite elements, or the presumed error by the difference in a stability analysis formula.
As s result, it became clear change of effect presumption of countermeasure by the presumed error of indefinite elements becomes larger as a scale of landslide is small, the influence of the indefinite elements increases by using countermeasure together, and the landslides which's evaluation value of countermeasure effect using the Fellenius method are bigger than using the simple Janbu method are more than 60% of whole.