To prevent breakage of a sealing device having a main sealing body installed within an annular installing groove (31) formed in one member (3) to slidably and concentrically contact with the other member (2) and a backup ring (12) arranged at a non-sealing space side of said main sealing body, a height (h1±Δh1) of said backup ring (12) in a facing direction of a bottom surface (31a) of said installing groove (31) to the other member (2) is greater than a facing distance (L±ΔL) between the bottom surface (31a) and the other member (2), and said backup ring (12) has a cross-sectional shape which is symmetrical in a thickness direction thereof, and has an easily compressible part (12a) which can be compressed to the extent of the difference (x) between said height (h1±Δh1) and said facing distance (L±ΔL).