The result is reported herein of a depth transform algorithm application utilizing equi-traveltime planes in reflection seismic exploration for the purpose of knowing the structure configuration in front of the tunnel face in a tunnel excavation process. A vibration producing hole was provided 0.5m above the ground surface while a vibration receiving hole was provided at 1.5m above the ground surface, and the excavated lengths were 1.5m and 0.5m, respectively. There were 24 recording channels, from which inline offset records were taken. The on-site records were subjected to 100-1000Hz band pass filtering. The elastic wave propagation rate in the rock used for depth conversion was set at 400m/s in consideration of the distance and direct wave travel time from the vibration source to the vibration receiving point. As the result of study, weak layers were found in the neighborhood of 230-240m, 260-270m, and 280-290m. This method requires less core memory for calculation and completes calculation in a shorter period of time, which enables a conclusion that it will be effective in grasping the 3-dimensional structure in front of the tunnel face in the working scene. 4 refs., 7 figs.