To provide an aerator capable of taking in the open air into a chassis without opening a door as needed while usually assuring airtightness of the chassis.

The aerator connected with a nipple passing through a wall surface of the chassis includes: a movable part comprising a washer plate, an elastic part provided on one of sides of the washer plate and an elastic member provided on a side other than the elastic part of the washer plate; an outline located along an outer periphery of the movable part and formed to be cylindrical; and an end cap screwed with an end of the outline and fixed thereto. The outline includes an aerating filter, an end cap thread located at one of ends of the aerating filter and having a thread inside, and a nipple screw located at the other end of the aerating filter and having a thread inside. While the aerator is mounted on the nipple, the elastic part of the movable part and the end of the nipple are joined while keeping constant contact pressure by a repulsive force of the elastic member of the movable part.