To reduce a shaft insertion load and ensure a sealing function, a carbon dioxide gas sealing device has a seal ring provided in a mounting groove having a tapered groove bottom, a backup ring having a tapered portion corresponding to the tapered groove bottom, and a gap provided between the side wall surface at the low pressure side of the mounting groove and a side end surface at the lower pressure side of the backup ring, a tapered surface that approaches the peripheral surface of a housing from the side end surface on the high pressure side toward the lower pressure side is provided on the peripheral surface on the side opposite to the tapered portion and the tapered portion has two-stage tapers.