a housing provided with a liquid inlet, a liquid outlet and an air passageway, liquid made to flow from said liquid inlet to said liquid outlet in said housing;

a sealing base installed in said air passageway of said housing and having an inner chamber and an outer-air inlet communicating with said inner chamber, an annular sealing surface formed around said outer-air inlet;

a floating cup installed in said inner chamber of said sealing base and provided with an annular flange extending radially outward and spaced apart equidistantly from its outer surface, said annular flange having plural air paths for air to flow up and down therethrough, wherein said floating cup is further provided with a closed air chamber in its interior, and wherein the floating cup consists of a cup body and a bottom cap, said cup body has an inner space opening downward, said bottom cap seals a bottom end of said cup body, and said closed air chamber is defined between said cup body and said bottom cap;

a sealing ring fitted around an upper portion of said floating cup and floating together with said floating cup to contact closely said annular sealing surface of said sealing base as to seal up said outer-air inlet, said sealing ring moving down together with said floating cup as to open said outer-air inlet when a vacuum condition happens in said device; and

plural plate ribs are provided to extend down from a preset portion of said annular flange, and an open area is formed between every two of said plate ribs for gas and liquid to flow through.