desirable to locate the lower edge 33 of the curved surface 7 at a position lower than the lower edge section 34 of the rotary disc 1 the uprising air stream 11 turns into an uprising swirling air stream 13 which is lead out from the air exhaust port 9 into the main duct 10 together with fly waste generated inside the yarn twisting space 12 the other end of the main duct 10 is connected to a suction device not shown the fly waste is collected and piled up at a predetermined location by means of suction of the suction device the lower section of the main duct 10 is formed to include an l shaped belt cover 14 which covers up a belt running chamber 15 to muffle noises the numeral 16 is a packing which interlockes the air exhaust port 9 and the main duct 10 the upper section of the balloon cover is cut off to define a slant truncated open surface 17 which is covered by a cover lid 18 to render the twisting yarn space airtight the open surface 17 is shaped as a slant surface such that the front section thereof is lower than the rear section and the open area thereof is larger than.