To provide a device and a method for reinforcing support legs of a self-elevating platform capable of reinforcing the support legs according to the conditions such as the water depth while requesting no large apparatus.

The self-elevating platform includes a platform body 2 movable on the water, and a plurality of supporting legs 3, 3 which are relatively movable in the vertical direction with respect to the platform body 2. A supporting leg reinforcing member 10 for connecting the supporting legs 3, 3 in a relatively non-movable manner in the horizontal direction is provided on each of the supporting legs 3, etc. in a relatively movable manner in the vertical direction. In the supporting leg reinforcing device of the self-elevating platform 1 in which the supporting leg reinforcing member 10 is supported by each of the supporting legs 3, etc. in a relatively non-movable manner in the vertical direction at the desired height position in water, a hollow floating part is provided to the supporting leg reinforcing member 10, the supporting leg reinforcing member 10 is vertically moved by adjusting buoyancy of the floating part by filling/discharging water in/from the floating part.