Abstract: An air supplying devices of the type which can be disposed properly on the stairwell of a building is provided. The air supplying device comprises an air duct which defines an inlet and an outlet which are directed downward and are provided with protecting cap at the opening thereof. The inlet of the air duct being disposed at outer wall of the building in a position between first and second floor. The outlet is disposed at top of the building. The air duct is attached to the handrail of the stairwell or serves as a handrail. A plurality of branch ducts are connected to the air duct thereof and which are spaced from each other with a predetermined distance. Each of the branch ducts is provided with a neck portion having a plastic ball seated thereof in normally and which is lifted when a negative pressure is applied. The opening of the branch duct being suitably directed and protected with a barrier grid.