PURPOSE: To enable to operate economically, unnecesitate the arranging of ducts in the ceiling and lower the ceiling height of a building, by a method wherein the opening of each blow-off port is controlled by detecting the temperature in each room, and the quantity of air being blown off is controlled in accordance with load in the room by controlling the rotating speed of a blower motor.

CONSTITUTION: An air-supplying duct 4 in the plenum chamber 3 is provided with a main blow-off port 5 for blowing off conditioning air toward a floor 1, and a variable air qnantity blow-off port 6 is provided in the ceiling 2. A thermostat 8 is provided in the interior of each of rooms 7A, 7D, and is connected to a small-type motor 9 provided at the port 6 by an instrumentation wiring 10. An instrumentation wiring 12 is extended from the motor 9 to a power-controlling panel 11, and a power wiring 15 is connected from the panel 11 to a motor 14 for an air conditioner 13. The opening of each variable air quantity blow-off port 6 is detected through a limit switch or the angle of rotation of a small gear 22 or the like, and the rotating speed of the motor 14 is controlled through the panel 11 in accordance with the detected opening, thereby controlling the air quantity of the air conditioner 13.