NOVELTY - The apparatus (A) has supporting legs (3) relatively moved up and down with respect to a ship main body (2), The supporting legs relatively support a supporting leg reinforcement component (10) immovably up and down in a desired height position in water. The supporting leg reinforcement component is moved along up and down direction by equipping the supporting leg reinforcement component with a hollow floating-body part, irrigate-draining in a floating-body part and adjusting buoyancy of the floating-body part.    USE - Apparatus for reinforcing a supporting leg of a self-raisable/lowerable ship i.e. self elevating platform (SEP), during sea installations construction i.e. wind power generation installation, in a deep-water place.    ADVANTAGE - The supporting leg reinforcement component is moved along up and down direction by equipping the supporting leg reinforcement component with the hollow floating-body part, irrigate-draining in the floating-body part and adjusting buoyancy of the floating-body part so as to avoid requirement of large-scale installation and reinforce the supporting legs according to conditions such as depth of water, in a convenient manner. The apparatus adopts a position control part connected with the supporting leg reinforcement component, and the lower end part of the position control part is easily drawn out, so that high vertical motion can be provided to the ship main body. The apparatus guides the supporting leg reinforcement component to the position control part by controlling a position of the supporting leg reinforcement component based on feed amount of the position control part, so that positioning of the supporting leg reinforcement component can be easily performed.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a self-raisable/lowerable ship supporting leg reinforcement method.    DESCRIPTION OF DRAWING(S) - The drawing shows a front view of a supporting leg reinforcement component.    Supporting leg reinforcing apparatus (A)    Self-raisable/lowerable ship (1)    Ship main body (2)    Supporting legs (3)    Supporting leg reinforcement component (10)    Slide part (12)