PURPOSE: To perform an effective dredging operation in accordance with the rising and falling of a hard sea bed and the water depth by arranging two cutters rotary in opposite direction from each other.

CONSTITUTION: A ladder 1 is secured elevationally and pivotably to the dredger V at a pivot point 2. The ladder 1 has pivoted at the tip end a hanger 3. A cutter supporter 8 is secured pivotably through a shaft 9 to the lower end of the hanger 3 so as to swing in the digging direction. On the supporter 8 are arranged two same size cutters 10, 10', which are mounted on the rear end of their shafts 11, 11' with drive means 12, 12' which turns in the opposite directions from each other. Near the cutters 10, 10' are opened the inlet 14 of a suction pipe 13 through a flexible hose. Thus the cutters 10, 10' are pivoted to the hanger 3 to move swinging in the digging direction and turning in the opposite direction from each other, and thereby they only receive equally divided load. Thus an effective dredging operation can be performed.