To reduce a shaft insertion load and prevent extrusion of a seal ring to the low pressure side, a sealing device has a seal ring in an installation groove formed on a shaft to have a tapered groove bottom section and a backup ring having a tapered section corresponding to the tapered groove bottom section, a gap is provided between a side wall surface at the low-pressure side of the mounting groove and a side end surface of the backup ring, a peripheral surface of the backup ring at the opposite side of the tapered section has a tapered surface which comes closer to a peripheral surface of a housing at the low-pressure side, and the tapered section is formed to have a two-staged taper.