To provide a method of easily connecting a small working ship having a ballast water washing device to a ballast water outboard discharge port of a main vessel even on the sea in heavy weather, a mechanism for operating it, and a connecting device therefor.

A hanger for the connecting device having the connecting device is delivered from the working ship side to the main vessel side, effect of the opposite movements of the main vessel and the working ship due to ocean wave is cut off, and a transfer pipe is a flexible hose. A suspension rope of a hose davit for delivery is loosened to the extent of movement of the main vessel and the working ship or more upon recognizing the delivery. A remotely operable actuator (hydraulic cylinder) is mounted to the connecting device including the hanger for the connecting device. The main vessel is provided with a bell-mouth for connection and a main vessel mounting hook for mounting the hanger for the connecting device. The small working ship is equipped with the ballast water washing device, the hose davit for mounting the connecting device to the main vessel, and a drive source, an operation panel and the like for a remote operation.