The cap comprises a cap body and a clip mounted onto the cap body. A portion having a small sectional surface, being smaller than that of an adjacent portion is provided at the forward end portion of the cap body. An annular mounting portion fitted over the portion having the small sectional surface of the cap body is provided on the clip. An air path allowing a space formed at the forward end to communicate a space formed between the main body of the clip and the cap body at the tail end is formed between this mounting portion and the having the small sectional surface. Alternatively, a cap with a first air path at the forward end communicating with a space at the tail end thereof is formed between the mounting portion and the portion having the small sectional surface. A second air path allowing the space formed at the forward end to communicate with the space formed at the tail end is formed at a position wehre the second air path is connected to the first air path in the cap body. @(16pp Dwg.No.1/10)@
