In the area of Yon-ch'on, it was rained some heavy rainfall amount of 171.4mm from 2001/7/29 to 2001/7/30, and the excavated slope was failured. To investigate the cause of that failure, geological surveys, investigation of geology and the influence of heavy rainfalls on the failure of the excavated slope were studied. Based on the results of those investigations, the failure of the excavated slope was mainly affected by the daily rainfall and the maximum hourly rainfall in the failure day.