Subterranean termites construct underground tunnels, tens to hundreds of feet, to reach feeding sites and to transport food items to their nest. To ensure a high rate food return to the nest, an optimized tunnel should be constructed. We found that termites (Coptotermes formosanus Shiraki) fill the corner of a bent tunnel with soil particles excavated from tunnel tip where the tunnel is advanced. The corner-filling behavior, eventually, smooth a sharp corner. In the present study, we showed that the corner-filling behavior can play an important role in improving the tunnel traffic efficiency. We compared the termites’ time spent for passing corners between a right-angled flat tip (RA-corner), corresponding to the sharp corner, and a rounded tip (R-corner) corresponding to the smooth-rounded corner. The passing time in the R-corner was significantly shorter than in the RA-corner. In addition, tunnel width effect was discussed in terms of individual movement.