In semi-enclosed water areas, aquaculture has been performed actively in many places in Japan by utilizing their calm environment. In case of overproduction of culturing, however, aggravation of water quality environment, such as eutrophication and appearance of anoxic water, is inherent problem in semi-enclosed areas. Nagatura-ura Lagoon, the study area of the present study, is also one of the typical semi-enclosed brackish-water areas. Nagatsura-ura Lagoon has been used mostly for cultivating oyster for these several decades. In the recent years, however, the appearance of anoxic water causes severe damage to the fishery industry. In this study, filed observation has been carried out during summer of 2003. The salinity, water temperature, dissoloved oxygen and water level were measured in the study area. Through these measurements, the characteristics of water quality change in response to tidal variation were investigated to clarify the mechanism of sea water exchange in the lagoon area.