The seasonal variation of water quality at the honjyo region in the brackish Lake Nakaumi was examined on the basis of the monthly data from January in 1997 to December in 1998. A significant negative correlation between salinity in the surface water at the center of Lake Nakaumi and the cumulative rainfall was found, but its correlation at Honjyo region was weak. The variation extent of salinity in the surface water was lower at the Honjyo region than at the center of Lake Nakaumi. In summer the concentrations of ammonium nitrogen phosphate phosphorus increased remarkably at the bottom layer of dredging station in the Honjyo region. Total nitrogen concentration at the Honjyo region increased after long rainy terms, 1997. The Chl.a concentration increased at the same time, reaching the high concentration of 76μg/l. Transparency at the Honjyo region was much higher than that at the center of Lake Nakaumi. The reason for this interesting phenomenon was also discussed.