A drain pipe 54 includes an upper end portion 54a which is connected to a concave 38a of a leading end 38 of an exhaust tube 34, and a lower end portion 54b which is opened to the inside of a warm air furnace below the concave 38a. The lower end portion 54b is formed as a tubular portion where a membrane is formed at a plane orthogonal to an axis line 54d by a surface tension of a drain. A leading end 54e of the tubular portion is formed as an opening 54f with an area large enough such that the membrane is not generated by the surface tension of the drain.