PURPOSE: To enable multiple-stage control of the quantity of airflow by attaching an air permeable and various germ-barrier film to one or both of a cover unit and a body unit respectively having opening parts in which the vertical positions and the heights are respectively coincided between both the units in a state where the cover unit is set to the body unit and in which the widths and the horizontal positions are respectively partly coincided between both the units.

CONSTITUTION: In the middle part on the side wall 3 and 5 of 1 plane, 2 plane, 3 plane or 4 plane of a body unit 1 made of a transparent plastics and having a cross-section of a square with somewhat round corners and in the side walls 8 and 11 on 1 plane, 2 plane, 3 plane or 4 plane of a cover unit 6 having protruded ribs 7 as the periphery of the upper outer plane, opening part 3, 9, 5 and 12 having a width mutually different between both the units are made so that the heights between both the units may be coincided at the position 13 where the cover unit 6 is set to the body unit. The above-mentioned opening parts 3 and 5 or 9 and 12 are covered with an air- permeable and various germ-barrier film 10. The position where the cover unit 6 is set to the body unit 1 is changed 90°C so as to vary the overlap between the opening parts 9 and 12 of the cover 6 and the opening parts 3 and 5 of the body unit 1, thus controlling the quantity of airflow in four stages.