An automatic aeration device includes a first floating roof, a second floating roof and a movable valve. The first floating roof has a through hole and a first enclosure. The second floating roof is disposed in the through hole and has a second enclosure. The movable valve is disposed above the first floating roof and the second floating roof and has a plate which has an outer wall and an inner wall. The outer wall is mounted in the first enclosure and has a plurality of first vent holes. The inner wall is mounted in the second enclosure and has a plurality of second vent holes. The automatic aeration device further includes a plurality of balls mounted between the first enclosure and the outer wall, and mounted between the second enclosure and the inner wall.

One embodiment relates to a valve device. The valve device includes: a valve case; a valve element slidably disposed within a valve chamber defined in the valve case; and a positive pressure spring configured to urge the valve element toward a valve seat in the valve case. The valve element includes an outer circumferential portion having a predetermined length extending along an inner circumference of the valve chamber. The outer circumferential portion includes plural opening portions. And, ribs project from the inner circumference of the valve chamber to enter into the opening portions, thereby limiting movement of the positive pressure spring in a radial direction.