PURPOSE: To improve the variability of airstream along the whole length of the blow-off port of the air curtain and uniform the airstream by a method wherein an airstream straightening plate, driving the box of the fan lengthwisely into two parts including the axial line of the axial flow fan, is provided in the device.

CONSTITUTION: The variabilities of airstreams at respective points along the whole length of the blow-off port 3 may be improved remarkably by a method wherein the air straightening plate 6, dividing the box 1 of the fan into two parts containing the axial line 5 of the axial fan 4, is provided to blow the airstream off as a curtain stream from the blow-off port 3.