an armrest is unable to rotate downward and is able to rotate upward within an adjustable angle range which is between an initial position of the armrest that is horizontal with respect to a seat and a position of the armrest raised by a specific angle;

when exceeding the adjustable angle range, the armrest is able to rotate upward to a stowed position and to rotate downward to the initial position; and

when the armrest returns to the initial position, a downward rotation thereof is disabled and an upward rotation thereof is enabled; and

wherein said armrest apparatus comprises:

a drum provided on the seat,

a casing that is provided on the armrest so as to be rotatable relative to the drum,

a coil spring with one end thereof fixed to the casing and wound around the drum and another end thereof being in a free state,

a cam formed in an end surface of the drum, said cam acting on the free end of the coil spring and displacing the free end in an axial direction and peripheral direction of the coil spring according to a relative angular displacement between the drum and the casing, and

a spring guide that acts on the free end of the coil spring and allows the displacement in the peripheral direction of the coil spring to be maintained and returned according to the relative angular displacement between the drum and the casing; and wherein

when the armrest is within the adjustable angle range from the initial position, the free end of the coil spring remains in the same cam-induced axial and peripheral displacement, and peripheral displacement of the free end of the coil spring is allowed by the spring guide so that the downward rotation of the armrest is disabled by the tendency of the coil spring to tighten and the upward rotation of the armrest is enabled by the tendency of the coil spring to loosen;

when the armrest exceeds the adjustable angle range, the free end of the coil spring is displaced in the axial direction and the peripheral direction by the cam, and displacement of the free end of the coil spring in the peripheral direction is maintained by the spring guide, so that the coil spring is kept in a constant loosened state and the armrest is able to rotate upward and downward; and

when the arm rest returns to the initial position, displacement of the free end of the coil spring in the axial direction and the peripheral direction is eliminated by the cam, and displacement of the free end of the coil spring is in the peripheral direction is allowed to be returned by the spring guide.