Many conventional stability evaluation methods are based on techniques and knowhows of specialist of geology and civil engineering. In this paper, we deals with a new stability evaluation method in which the black box of the judgements by the specialists is solved and is opened. At first, the site-investigations and inspections are exceeded in object to both of downfall-type rock fall and separation-type rock fall/rock collapse type at hard rock slopes along railways. Some quantification analyses using the data are also exceeded for each of the both types. On the base of the results, we extract a critical primary causes (CPCs), which are fundamental factors in relation to hard rock slope collapses, and distill instability factors for past, presence and future. At last we present a new stability evaluation method for hard rock slopes in consideration with the above conclusions.