Abstract: A device for locking a platform of an offshore structure wherein each of legs vertically rest on the sea floor and is provided with a rack. The platform in adapted to move upward or downward along the legs and is provided with chocks which engage with the racks, respectively, of the legs. The teeth of the chock has a whole depth deeper than that of the teeth of the rack so that when the chock engages with the rack, the deddendums of the chock is caused to deflect themselves respectively about the deddendums, which are made into engagement with the rack, so as to follow the displacement of the platform in the direction of gravity.