<p>A dust-proof boot (3) comprising an internal engaging (7) ring extended from one end of an expansible bellows portion (6) and an external engaging ring (8) provided upwardly thereof and capable of being spread is provided at an extended base of the internal engaging ring (7) with an air flow port (9) for guiding pressure from the inside of the bellows portion (6) into the internal engaging ring (7). In mounting the dust-proof boot (3) on a support (32) provided with a guide hole (33), the internal engaging ring (7) of the dust-proof boot (3) is fitted into an internal groove (4) of the support (32), after which compressed air is supplied from the interior of the bellows whereby the compressed air spreads the external engaging ring (8) via the air flow port (9) and the external engaging ring (8) is fitted into an external groove (5). <IMAGE></p>
