To provide a cover member which prevents insects or the like or impurities like dust or the like together with the outside air from entering indoors through a ventilation hole formed in a wall body of a building to take the outside air in.

An elastic cylinder 5 made of a foamed synthetic resin is fixed to a rear face of a ring-shaped cover body 1 in an integrating manner. The elastic cylinder 5 is tightly brought into contact with a wall surface around the opening end of the ventilation hole in a state of elastic compression by fitting a plurality of coil springs 9 to the cover body 1 and by engaging hooks 8 mounted on front ends of the coil springs 9 with the opening end of the ventilation hole. Further, a filter 3 covering the opening end of the ventilation hole is laid on a front face of the cover body 1 and is fixed to the cover body 1 by a ring-shaped filter holder 4. Thus, the elastic cylinder 5 is always brought in elastically tight contact with the wall face and prevents the insects or impurities together with the outside air from entering from the outside.