On the occasion of the stability evaluation to the unstable rock blocks in rock cliff which exist in the upper part of a tunnel entrance, many judgments based on the displacement behavior of the rock blocks are made. It is known that the time from the generation of the displacement to the collapse of the rock block is very short. Therefore, it is hard to catch a sign of collapse. The authors measured the dynamic characteristic of the rock blocks by the seismometers as well as the vertical

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... cement measurement of the rock blocks by the displacement meters. As a result, it was confirmed that the dynamic characteristics of the rock blocks change before the displacement occurs.