In tunnel construction, overcut along with undercut estimation is one of the most important factors to be considered before proceeding to the next operation. It is currently analyzed based on sparsely sampled points surveyed using a total station, but not much time is allowed for surveying and analyzing for economic reasons. A fast and automated method is presented to extract dense tunnel cross-sections using Terrestrial Laser Scanner (TLS) data. A 3D point cloud acquired from the TLS is converted to a two-dimensional planar image and skeletonized to estimate the tunnel centerline. Cross-sections are extracted orthogonal to the centerline. To evaluate the performance of the proposed method, it was applied to actual tunnel data and compared with the results from a conventional method using a total station. In the results, the cross-sections were extracted at center points corresponding to those of the conventional method. The proposed method proved itself to have advantages in terms of its ability to offer a detailed description and improve the efficiency of the processing time.