PURPOSE: To prevent any breakdown from occurring, by setting a mooring buoy extending sideward to a buoyant body in a vertical direction to the seabed rockably, while engaging a mooring rope to the buoyant body via a movable buoy installed in this mooring buoy so movably in an up-and-down motion, and thereby absorbing an increment in mooring rope tension with inclination of the mooring buoy and the vertical motion of the movable buoy.

CONSTITUTION: A mooring buoy 1 extending sideward to a buoyant body A in a vertical direction is installed rockably via a rope 2 engaged to the seabed B. A movable buoy 3 being free of an up-and-down motion is installed along an upper guide rod 12 of this mooring buoy 1, and one end of a mooring rope 4, whose other end is engaged to the buoyant body A, is engaged to the movable buoy 3. IN order to cope with displacement in the buoyant body due to waves, etc., a state of tension in the mooring rope 4 is absorbed with vertical movements of both the mooring buoy 1 and the movable buoy 3, thus any breakdown can be prevented.