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In Japan, many diadromous aquatics migrate for reproduction and/or growth between the seas and the rivers during their life history. They include many commercially important species for river fisheries, e.g. the ayu fish Plecoglossus altivelis, the Japanese eel Anguilla japonica, the Japanese mitten crab Eriocheir japonica, and so on. Many artificial structures like weirs and dams are constructed in the rivers of Japan and they become barrier walls that block migration. Most of the structures have fishways for big fish, e.g. salmon and trout, but those fishways are not always suitable for small diadromous animals. I have conducted a joint research with Yamaguchi Prefecture to improve the existing fishways with a low-cost and simple method of construction. The “Mizube-no-Kowaza” fishway is a slope type fishway having protruding boulders on its bed. The term of “Mizube-no-Kowaza” means a collection of small-scale devices for the improvement of river structures. We aim to connect the upper stream with the lower stream for aquatic organism’s migration by adopting the “Mizube-no-Kowaza” method. It can be constructed at low cost because of its simple structure. The “Mizube-no-Kowaza” fishway is effective in increasing wild resource of river dwellers by promoting networks among habitats isolated by manmade structures.