Abstract: In a lockable folding joint of a ladder having two arms with their flat heads pivoted to one another, a detent block used to engage one of spaced notches of one of the flat heads is held resiliently and firmly by an improved mechanism. The mechanism includes a one-piece curved rod spring which has two opposite arc-shaped rod portions whose one ends are attached to the detent block, two coiled portions formed at another ends of the arc-shaped rod portions and a U-shaped rod portion extending from the coiled portions. A spring plate has a rolled end pivoted to one of the arm with the same pivot pin as the coiled portions. The U-shaped portion depresses the spring plate so that the hook end of the spring plate is engaged tightly with the detent block.