Subterranean termites build extensive underground galleries that consist of elaborate tunnels and channels to forage for food resources. The changes in tunnel width along the length of the tunnel are related to both biotic (e.g., termite activity) and abiotic factors (e.g., soil density). Termites transport food through the tunnels from food sources to their nest. Thus, understanding the relationship between traveling behavior in the tunnels and changing width is important to comprehend the stability of the termite ecosystem. In the present study, we explored the traveling behavior of termites in terms of ment efficiency, where the ment efficiency was defined as the time (s) needed for a termite to pass through a tunnel. To do so, we designed artificial tunnels with linearly changing width in a two-dimensional arena. The tunnel widths, W1 (for the entrance) and W2 (for the exit), were 2, 3, 4, 5, and 6 mm. We systematically observed the traveling behavior of the termites Reticulitermes speratus kyushuensis Morimoto (Isoptera: Rhinotermitidae) in the artificial tunnels and measured s. The value of s increased with the increase of W2, regardless of W1. s was longer in the case of W1W2. The experimental results can be explained by behavioral differences observed in each case. The implications of the findings are briefly discussed in relation to termite foraging efficiency and the development of individual-based models for the construction of termite tunnels.