Abstract: An offshore platform provided with several supporting legs ascending and descending vertically and installed in a specified place on the sea by lowering the supporting legs onto the seabed after being towed to the specified place. Each of the supporting legs has on its side racks extending longitudinally. The supporting legs are lifted and lowered by driving pinions that engage with these racks. The offshore platform can be held immovably on the sea by fixing the meshing of the racks with the pinions, which is accomplished by braking pinion drives, when the supporting legs have reached the seabed. This offshore platform is also provided with stoppers that can engage with and disengage from the racks through the lateral movement thereof toward and away from the racks. The supporting legs are held by both the pinions and the stoppers.