Geological and geophysical surveys were performed to examined the cause of the rock-mass slide occurred at the opening site of the Wanglim tunnel of the Seoul-Pusan High Speed Railway. The results of geophygical survey and geometrical analysis for the geological structures indicate that the rock-mass slide was triggered by the heavy rainfall which increases the groundwater level and results in high pore pressure in the N60$^{circ}$ E-trending major fractured zone of the slope, and that the foliation dipping toward the vertical open face of the slope acted as a main sliding plane during movement.