To improve the releasing efficiency by providing an outer circumferential face on an upper side float body of the degree of abutting on the inner wall of an inner case part, circumferentially providing vertical recessed parts having proper space on the outer circumferential face between it and the inner wall at proper intervals.

When air in a duct A rises to flow in an inner case part from through holes 13, 24, in the case of a large quantity of the flow-in air, the upper and lower float bodies 3, 4 together are lowered, the upper and lower float bodies are both lowered, the large valve port 52 of a cover body part 5 is opened, and air is release from the large valve port 52 to the open air. Further in this case, when a large quantity of air is at once and continuously release, high speed air flow toward the large valve port 52 is generated. However the high speed flow is quickly led to the large valve port 53 by the projecting part of an upper side float body, and because air in the lower space of the upper side float body 3 passes through to transfer to the upper space, the float bodies 3, 4 are floated up not to become an obstructing factor for air relase, and efficient air bleeding is performed.