PURPOSE: To handle a large boat with a device nearly equal in size to the device for a small boat.

CONSTITUTION: The right and left girders 2 are arranged in parallel with the sea side at a place with a required height on a quay 1. The girders 2 are supported by expansion legs 9. An upper leg member 13 and a lower leg member 14 are vertically slidably combined to form the expansion leg 9. An expansion actuator is provided between the upper leg member 13 and the lower leg member 14. The expansion leg 9 is expanded or shrunk by the operation of the expansion actuator to change the height of the girder 2. The height of the girder 2 is maintained by a stopper pin insertion type position fixing mechanism.