Groundwater level change observed prior to the 2000 eruption of Usu volcano demonstrates that the failure in rock obeys a scaling rule. Three months before the eruption, the observed groundwater level perturbedly decreased around a curve like a power law, which appeared as an oscillation pattern like self-similarity. The fluctuation in groundwater level is considered to be the inherent critical phenomenon of crustal rock prior to rock failure. The critical phenomenon offers the potential for prediction of failure in rock. The failure point in rock can be foreseen, more than one month before the rock failure.