PURPOSE: To permit the control of ventilation of a clean room across a wide range by controlling the flow of air through respective individual ceiling panels.

CONSTITUTION: Air is supplied from a blower 11 into a clean room 10 with a controllable speed through a variable damper mechanism. The ceiling of a room 16, to which the air is supplied, is constituted of a plurality of panels 14 consisting of a molecular filter substance. Some panels 14 are provided with an air damper consisting of approximated first and second perforated plates. The first perforated plate is fixed and the second perforated plate can be moved in parallel to the first perforated plate. The second perforated plate is moved with respect to the first perforated plate whereby respective bore holes are opened and/or closed selectively and air, passing through respective panels 14, is controlled. The flow of air, passing through the whole of the room 16, is controlled by the variable characteristics of flow of air of the blower 11. According to this method, ventilation of the clean room 10 can be controlled across a wide range.