PURPOSE: To absorb varying external forces by moving a float body by a method in which a float body on the sea surface is moored in a half-submerged state by a vertical mooring material and skew mooring lines and an intermediate weight landed on the seabed is provided on the way of the skew mooring lines.

CONSTITUTION: The float body 1 of a marine structure on the sea surface 5 is moored in a half-submerged and excess bouyancy-retained state by means of a vertical mooring material 2 and skew mooring lines 6. The skew mooring line 6 is made up of a skew mooring material 7, a horizontal mooring material 9 whose one end is connected with a submarine anchor device 10 on the seabed 4, and an intermediate weight 8 provided in a liftable manner from the seabed between the skew mooring material 7 and the horizontal mooring material 9. When the horizontal external force of waves acts on the float body 1 and the external force exceeds a given value, the intermediate weight 8 floats up from the seabed and the spring constant of the mooring becomes smaller. Accordingly, varying external forces are absorbed by the moving of the float body 1 and the direct sharing of the external forces by the mooring material is prevented. The size of the mooring materials can thus be miniaturized.