PURPOSE: To raise the efficiency of operations by providing a device by which a grab bucket or rock drill is connected through a cable to a crane and the grab bucket, etc., is held over the horizontal and all directions near to the position of dredging work.

CONSTITUTION: A connector 108 joined with a crane K is driven by wires 111a and 111b to move it to a desired position. When a grab bucket or rock drill 105 begins to lower by the releasing of a support winch brake, a moving guide 115 surpasses a hanging weight 128 by movement of bucket, etc., 115. When the guide 115 reaches a stopper 119 on the lower end of a guide pole 102, the release lever 118 of the guide 115 serves to release the connection with a spigot 132 with bucket by the inertia of the release lever terminal weight 128 and also by the release lever release stopper 119, and the bucket, etc., 105 goes down into sea water. Excavation is further made by bucket, etc., 105, the bucket, etc., 105 is raised, and excavated soil is discharged. The same operations are repeated.