A sealing structure comprising: a shaft provided with an annular groove formed on an outer peripheral; a housing having a shaft hole through which the shaft is inserted; a seal ring formed of a rubber-like elastic body that is installed in the annular groove and that seals an annular gap between the shaft and the shaft hole; and a resin backup ring installed adjacently to the seal ring and at a position further toward a low pressure side than the sealing ring, wherein on a low pressure side of a groove bottom surface of the annular groove, a tapered surface is formed which has a diameter increasing toward the low pressure side, a cut portion that is cut in a planar shape is formed in the backup ring at one position in a circumferential direction, the cut portion is configured such that a first cutting line formed on a surface on a high pressure side of the backup ring is parallel to a second cutting line formed on a surface on a low pressure side of the backup ring and such that the cut surface is oblique to a central axis of the backup ring, and an acute angle of angles between the surface on the high pressure side of the backup ring and the cut surface is set to not less than 15° and not more than 30°, and the backup ring is formed of a resin material having Rockwell hardness of 100 or less, durometer hardness of 70 or less, and elastic modulus of 1.0 GPa or less, wherein an inner peripheral surface of the backup ring is formed of a tapered surface that has a larger taper angle than the tapered surface formed on the groove bottom surface and that has a diameter increasing toward the low pressure side so that substantial line contact is enabled between an annular edge portion on the high pressure side of the inner peripheral surface of the backup ring and the annular groove, and that the backup ring is disposed at a position where the annular edge portion on the high pressure side of the inner peripheral surface of the backup ring is slidable with respect to the tapered surface formed on the groove bottom surface. a shaft provided with an annular groove formed on an outer peripheral; a housing having a shaft hole through which the shaft is inserted; a seal ring formed of a rubber-like elastic body that is installed in the annular groove and that seals an annular gap between the shaft and the shaft hole; and a resin backup ring installed adjacently to the seal ring and at a position further toward a low pressure side than the sealing ring, wherein on a low pressure side of a groove bottom surface of the annular groove, a tapered surface is formed which has a diameter increasing toward the low pressure side, a cut portion that is cut in a planar shape is formed in the backup ring at one position in a circumferential direction, the cut portion is configured such that a first cutting line formed on a surface on a high pressure side of the backup ring is parallel to a second cutting line formed on a surface on a low pressure side of the backup ring and such that the cut surface is oblique to a central axis of the backup ring, and an acute angle of angles between the surface on the high pressure side of the backup ring and the cut surface is set to not less than 15° and not more than 30°, and the backup ring is formed of a resin material having Rockwell hardness of 100 or less, durometer hardness of 70 or less, and elastic modulus of 1.0 GPa or less, wherein an inner peripheral surface of the backup ring is formed of a tapered surface that has a larger taper angle than the tapered surface formed on the groove bottom surface and that has a diameter increasing toward the low pressure side so that substantial line contact is enabled between an annular edge portion on the high pressure side of the inner peripheral surface of the backup ring and the annular groove, and that the backup ring is disposed at a position where the annular edge portion on the high pressure side of the inner peripheral surface of the backup ring is slidable with respect to the tapered surface formed on the groove bottom surface.