a front duct configured to change a discharging direction of an air stream horizontally and vertically;

a rear duct connected to a rear side of the front duct and a damper hinged to an inside of the rear duct and selectively blocking the air stream;

a rotatable knob coupled to a side of the front duct, formed with a hole at a side thereof, and configured to drive the damper; and

an angle-restricting device including a cylindrical housing integrally connected to a portion of the front duct and extending generally radially within the hole of the knob, the angle-restricting device elastically radially movable to restrict a rotation angle of the knob,

wherein the angle-restricting device includes a stopper installed in an open end of the cylindrical housing, the stopper configured to elastically move around an inner circumference of the hole as the knob is rotated, a pair of spaced stopper recesses formed at the inner circumference of the hole to receive the stopper as the stopper moves around the inner circumference of the hole to detect whether the damper is opened or closed, and the stopper is configured to move outwardly in a radial direction of the cylindrical housing when the stopper is received by the spaced stopper recesses, and

the hole is provided with a substantial cylindrical space between the inner circumference thereof and the portion of the front duct where the cylindrical housing is integrally connected, when the angle restricting device is assembled

wherein the cylindrical housing connects with said inner circumference of the angle restricting device only by the stopper.