Recently, in Japan, it is found that harmful sludge, which contains heavy metals and dioxins, has accumulated in the harbor or river mouth faced an industrial area. It is expected that these harmful matters have a bad influence on the fish and shellfishes, and it has been a serious problem. Therefore, when removing this sludge, it is very important that the turbidity is not generated as much as possible for preventing secondary pollution. In this study, in order to investigate the relationship between operating conditions and the turbidity cased by dredging using closed type grab bucket at the closed water surrounded by revetment. In the field test, closed type grab bucket was used. Vertical distribution of turbidity was observed continuously by the turbidity-meters that were installed in the six perpendicular directions at the grab bucket dredger about 10m away from center of the bucket. Moreover, the depth, the plane position, and opening degree of the bucket were measured in order to know the relation between turbidity and motion of bucket. The observation results indicated that there was a correlation between the speed of lowering bucket and the turbidity at the lower layer.