a leg unit;

a seat unit supported on said leg unit;

a pivot unit including two pivot joints secured to said leg unit; and

a backrest unit including two opposite backrest rods that are pivoted to said leg unit through said pivot joints, respectively, said backrest unit being rotatable relative to said leg unit between a first folded position, in which said backrest unit overlaps said seat unit, and a first extended position, in which said backrest unit stands upright from said seat unit,

wherein each of said pivot joints includes a generally U-shaped bracket plate and a pivot pin, said U-shaped bracket plate having two opposite arm portions and a base portion interconnecting said arm portions, each of said backrest rods having a portion sandwiched between said arm portions of said U-shaped bracket plate of a respective one of said pivot joints, said pivot pin of each of said pivot joints extending through said arm portions of said U-shaped bracket plate of a respective one of said pivot joints and said portion of a respective one of said backrest rods, each of said backrest rods being blocked by said base portion of said U-shaped bracket plate of a respective one of said pivot joints when said backrest unit is disposed at the first extended position, and

wherein each of said pivot joints further includes a bracket sleeve that is sleeved on said bracket plate of a respective one of said pivot joints, and that is formed with two opposite retaining round protrusions for holding releasably a respective one of said backrest rods when said backrest unit is disposed at the first extended position.