Investigation and estimation of disasters by rock fall are difficult as they involve a combination of various primary causes and inducements. Therefore, one of the most important subjects for the engineers who take charge of practical work is that they have to know causes and characteristics of the disasters by rock fall in the past, keep their eyes on the conditions and changes of environment of slopes. This paper reports a decipherment of general concept of disasters by rock fall and a study on the investigation method for extraction of their primary cause and inducement in a limited area whose characteristic is strongly related to rock fall.