To increase corrosion resistance and always retain stable sealability by providing a back-up tube of a dust seal in a scroll fluid machine with a new structure back-up member replacing a conventional back-up tube in the back of or inside of a sliding member.

The ring shaped sliding member 6 is fitted in an annular groove 12 provided on facing surfaces of a fixed scroll 17 and a revolving scroll 21 to surround both engagement parts via a flexible gripping ring 4 of an open U-shaped cross section gripping O-ring 3 from outside by both side pieces.