Hydrogen sulfide concentrations in bottom water and surface sediment pore water were measured during an anoxic period (September 2003) in brackish Lake Nakaumi, Japan. Bottom water hydrogen sulfide concentrations were greatest (5-127μg-Sl^-1; except for artificially dredged area) in the southern part of the lake and the Yonago-Bay area, where the hypolimnion is most stagnant, whereas hydrogen sulfide from pore water was detected ( ～1-37 mg-Sl^-1) in a much wider area. This indicates that surface sediments may maintain anaerobic conditions long after the bottom water return to oxygen rich. Hydrogen sulfide was not detected in bottom and pore waters near the Sakai-Strait and Ohashi River, where seawater and river water flow into the lake. In comparison with two other Japanese brackish lakes (Lake Abashiri and Lake Asokai), Lake Nakaumi has a lower concentration of hydrogen sulfide. Lake Nakaumi is characterized by an unstable hypolimnion, in which anoxic conditions are easily mediated by an oxygen-rich tidal-induced inflow.