In 1991, several projects for improving fish's upstream migration have been taken place. It is the work of consultants to make efforts in the construction and planning of fishways, which have not only to be proper for the flowing characteristic conditions of the river under consideration, but also have to fit to the circumstances of weirs/dams and other constructions. Besides this it is also important to minimize the environmental impact. For this research during the pre- and post-reconstruction phase, the environmental change caused the Ono weir's fishway at Koya River (Yamaguchi Prefecture) was continuously monitored. The interesting results are presented in this paper.