PURPOSE: To increase the amount of blow-off air and obtain a sufficient air- conditioning capacity by a method wherein a resistant body, projected from the side wall of the main frame of the grille into the air path to form a vacuum area, is made movable while the resistant body is not projected into the air path when the maximum amount of blow-off air is set.

CONSTITUTION: The resistant bodies 20, 21 are rotatable about hinge pins 22, 23 equipped on the side walls 9, 10 of the main frame 5 of the grille. Link arms 24, 25, which are attached to the upper parts of the hinge pins 22, 23, are connected by a link rod 26 and one of the link arms 25 is connected to a vacuum actuator 28 for driving with a wire 27 while the link arm 25 is rotated by 90° from a position X, where the resistant bodies 20, 21 are projected opposingly into the air path 4, to the position Y, where the resistant bodies are located along the side walls 9, 10 and are not projecting into the air path 4. The vacuum actuator 28 is connected to a vacuum source through a solenoid valve 29 and is started or stopped by a control lever 30 controlling the amount of blow-off air of the air conditioner.