Macrobenthic animals were observed from 1997 to 1999 in relation to induction of tidal flow brought from a construction of water-way pipes connecting with North Channel and Honjo Area in Lake Nakaumi, Shimane Prefecture, Southwest Japan. Before the construction, abundance and diversity of the benthic animals were very poor on channel bottoms, except for on a shallow site. The benthic animals, however, recovered after the construction, especially at the bottom near to pipe openings. After the pipes were removed, abundance and diversity of the benthic animals became very poor again. These results indicate that increase of tidal flow contributes improvement of benthic environment, probably due to increase of seawater intrusion. They suggest that further increase of seawater intrusion leads improvement of bottom environment of Lake Nakaumi, but detail studies on benthic ecology are required to evaluate the effect of the seawater intrusion on the benthic community.