PURPOSE: To prevent the invasion of noise completely by a method wherein an air passage having a similar cross section is formed in a continuously bent shape so as not to provide a corner part and an inside wall is covered with a sound absorbent.

CONSTITUTION: Openings 2 and 2' of a cylindrical air vent facing toward an inside and outside of a house respectively are provided, into which an air vent passage forming material 3 is inserted. The air vent passage of the material 3 has a similar cross section area in any place, which is formed in a continuously bent shape so as not to provide a corner part and a sound absorbent 5 like a glass wool is adhered to an inside wall. The material 3 is formed of a metal, hard plastics and strengthened glass, and a reflector 4 is formed on a side edge of the outdoor opening 2. 8 is a hood and 9 is a grating.