(1-14 Bunkyo-machi, Nagasaki 852-8521, Japan) 2 M . Eng., Graduate Student, Graduate School of Marine Science and Eng., Nagasaki University 3Member of JSCE , M. Eng., Research Associate, Department of Civil Engineering, Nagasaki University 4Member of JSCE , M. Eng., Fukuoka Branch, CTI Engineering Co., Ltd.(CTI Fukuoka Building, 2-4-12 Daimyo, chuo-ku, Fukuoka 810-0041, Japan) 5Student Member of JSCE , Graduate Student, Graduate School of Engineering, Nagasaki University 6 Engineer , Nagasaki Prefectural Office (2-13 Yedo-machi, Nagasaki 850-8570, Japan)Aiming at the sound and sustainable water environment, river should be managed taking care of the environment in nature. Also in the Honmyo River, nature-friendly river work has been done from 1991, of which influence on river water should be estimated both on the quantitative and qualitative viewpoints.In this paper, diurnal change of water quality in a pool has been investigated based on a field observation in the Honmyo River. As one of the results, it makes clear that existence of pool gives lots of influences on river water qualitatively, even if its depth is not so deep. From now on, it seems to be more important that the restoration work will be executed as well as decrease of pollutant runoff from the watershed based on the results which has been and will be obtained through this kind of research.