Predicting the onset of slope failure or rock falls is important for occupational safety because they kill approximately ten to 20 workers every year in Japan. Approximately half of such victims are from slope failure during slope excavation and construction. In order to predict the time of slope failure during an excavation, a full-scale model slope was built, and the displacement of the slope surface was monitored during slope excavation. The surface displacement rapidly increased with the elapsed time after the excavation, and the relationship between the displacement and elapsed time included an exponential function just before collapse. Based on the results, by computing the inverse of the velocity of the slope surface displacement obtained in ten-second intervals, a warning signal can be provided five min before collapse.