Sealing device

A sealing device comprising:

• a main sealing body which is installed within an annular installing groove formed in a first member of two members concentrically arranged with each other and is slidably brought into close contact with a second member of the two members;
• and
• a backup ring made of a harder material than said main sealing body which is arranged at a non-sealing space side of said main sealing body within said installing groove,
• wherein a height of said backup ring in a perpendicular direction from a bottom surface of said installing groove to the second member is greater than a perpendicular distance between the bottom surface of said installing groove and the second member, and said backup ring has a cross sectional shape which is symmetrical about a radial axis located midway in an axial thickness of said backup ring, and has an easily compressible outer diameter curved portion corresponding to a collapsing margin of said backup ring and which can be compressed to the extent of the difference between said height and said perpendicular distance.