PURPOSE: To control the vertical movement of a gravity-type dolphin by a method in which a vertically movable inner tank is set in an outer tank set on the seabed, and a slab fixed to the upside of the inner tank and supported on the neck piercing the upper wall of the outer tank is connected to an object to be moored.

CONSTITUTION: A semi-dividing type concrete inner tank 5 is set in an outer concrete tank 4 set on the rubble mound 3 of the seabed in such a way as to enable sea water to move through the aperture 12 between the tanks 4 and 5 and also the tank 5 to move vertically. The tubular neck 10 is fixed to the upper end of the tank 5, and the slab 15 on the sea surface is supported through the upper wall of the tank 4. The slab 15 and an object 2 to be moored are connected through bolts 17 and a connector 18. The difference of ebb and flow can thus be absorbed and vertical rocking by tidal waves can also be absorbed.