This study focus on finding out circumstances of occurrence of hypoxic water and relations between density stratification and hypoxic water occurrence based on observed data during 1972 to 2002 in the interior area of Ariake Sea. Hypoxic water frequently occurred during every summer in the bottom layer at many observed stations. It was also found that dissolved oxygen concentration of hypoxic water in the bottom layer was closely related with the density difference between surface and bottom layers.