To hold existent structures (structures with a shallow O-ring groove) unchanged wherein an O-ring is mounted or demounted with ease, and facilitate the mounting or demounting work of the protective member itself so that it requires no special care while being mounted or demounted.

The protective member, which has an expandable notch 10, comprises a spring steel 11 self-biased into a constricted condition, and a hard rubber 12 glued to the external surface of the spring steel 11. Mounting the protective member with the notch 10 expanded on an O-ring disposed in a pin joint part provides easy mounting thereof owing to the self-biasing.