The function of a test structure for the environmental restoration in enclosed sea area was assessed by several field investigations. The objective of the structure is to activate material cycle through the foodweb of marine organisms feeding on suspended solid. Dissolved oxygen concentration in the bottom of structure was lower than 3mg/l in summer, because a number of mussels on the porous concrete had consumed oxygen. The mussels also had an effect on decreasing suspended solid, and concentration of suspended solid in the structure wasrelatively lower than the outside due to their feeding activities. Amount of deposited seston varied with seasons. Thedeposited seston contained faeces and pseudofaeces of mussels, indicating that suspended solid was transferred by mussel filteration onto the bottom of structure. However, since few numbers of deposit feeder inhabited the bottomof structure, deposited seston have accumulated and caused deterioration of sediment quality. Some ideas for improvement of the structure were proposed in the discussion.