PURPOSE: To prevent any breakdown from occurring, by installing a mooring rope whose one end is connected to a buoyant body and the other end is connected to a mooring buoy having the specified buoyancy, while absorbing an increment in mooring rope tension with a sink and float action.

CONSTITUTION: A guide 2 is installed in an intermediate sinker 1 attached to the seabed B via a rope 5 and a mooring rope 3 is guided and supported via this guide 2. One end of the mooring rope 3 is connected to a buoyant body A while the other end is connected to a mooring buoy 4 having the specified buoyancy. When tension in the mooring rope 3 varies as the buoyant body A shifts to some extent, the mooring buoy 4 sinks and floats according to this tension whereby a state of tension comes loose, thus any breakdown can be prevented.