A fire retardant board for an outer wall of building is provided to intercept the upper end of the balcony of the floor on which a fire is generated automatically or manually to limit fire damage to one floor. In the fire retardant board(A), a fixing plate(10) is fixed on the outer wall of a balcony with a fixing unit(20,20'). A first fire retardant plate(30) is coupled on the lower end of the fixing plate(10) with a hinge(H). A second fire retardant plate(40) is hinge-coupled on the upper end of the first fire retardant plate(30) to be folded. The first and second fire retardant plates(30,40) are folded to the side of the outer wall of the balcony, and then, the upper ends of the first and second fire retardant plates(30,40) are fixed with a holding cover(50) fixed on the outer wall(W) by soldering.