A clip (10) has a frame (31) and a flapper (32). Two dry bearings (37) are inserted into a hole (32d) formed in the flapper. Then an attaching shaft (35) is inserted from one of the holes (34a) in the frame (31) to the other through the dry bearings (37), so as to swingably attaching the flapper (32) to the frame (31). DAIDAYNE DDK 01 of DAIDO METAL Corp. is used as the dry bearing (37). A first and a second clip covers (51,52) are provided to cover the clips (10). Fresh air feeding pipes (55) are connected to the first and second clip covers (51,52). Fresh air is fed from the fresh air feeding pipes (55) toward the clips (10) through air outlets (56) provided in the first and second clip covers (51,52).