A sealing device that is adapted to be placed in an annular gap between two members (51, 52) facing each other, one of the two members (51, 52) being provided with a mounting groove which is a groove with a rectangular cross-section having an opposing-sealing-fluid-side side surface portion and a bottom surface portion, and the other one of the two members (51, 52) having an inner diameter, to seal in sealing fluid, the sealing device comprising: a seal ring that is, when Amounted in the mounting groove, in close contact with the other member; a relatively hard first back-up ring placed on an opposing sealing fluid side of the seal ring; and a relatively soft second back-up ring placed between the seal ring and the first back-up ring, wherein the first back-up ring has a shape having an end surface portion adapted to be in contact with the opposing-sealing-fluid-side side surface portion of the mounting groove, a peripheral surface portion adapted to face the other member, and an inclined surface portion intersecting with the end surface portion and the peripheral surface portion, the second back-up ring has a shape having an end surface portion in contact with the seal ring, a peripheral surface portion adapted to face the bottom surface portion of the mounting groove, and an inclined surface portion that is provided to correspond to the inclined surface portion of the first back-up ring, a contact surface portion that is a plane perpendicular to an axis of the second back-up ring and adapted to be in contact with the opposing-sealing-fluid-side side surface portion of the mounting groove is provided in an inner peripheral end of the inclined surface portion of the second back-up ring, characterised in that an inner diameter size of the first back-up ring is set to be larger than an outer diameter size of the contact surface portion of the second back-up ring, and an outer diameter size of the first back-up ring is set to be smaller than the inner diameter size of the other member, the inclined surface portion of the first back-up ring is a surface with a concave circular cross-section, the inclined surface portion of the second back-up ring is a surface with a convex circular cross-section, the end surface portion of the second back-up ring is a surface with a concave circular cross-section, and the seal ring is an O-ring. a seal ring that is, when Amounted in the mounting groove, in close contact with the other member; a relatively hard first back-up ring placed on an opposing sealing fluid side of the seal ring; and a relatively soft second back-up ring placed between the seal ring and the first back-up ring, wherein the first back-up ring has a shape having an end surface portion adapted to be in contact with the opposing-sealing-fluid-side side surface portion of the mounting groove, a peripheral surface portion adapted to face the other member, and an inclined surface portion intersecting with the end surface portion and the peripheral surface portion, the second back-up ring has a shape having an end surface portion in contact with the seal ring, a peripheral surface portion adapted to face the bottom surface portion of the mounting groove, and an inclined surface portion that is provided to correspond to the inclined surface portion of the first back-up ring, a contact surface portion that is a plane perpendicular to an axis of the second back-up ring and adapted to be in contact with the opposing-sealing-fluid-side side surface portion of the mounting groove is provided in an inner peripheral end of the inclined surface portion of the second back-up ring, characterised in that an inner diameter size of the first back-up ring is set to be larger than an outer diameter size of the contact surface portion of the second back-up ring, and an outer diameter size of the first back-up ring is set to be smaller than the inner diameter size of the other member, the inclined surface portion of the first back-up ring is a surface with a concave circular cross-section, the inclined surface portion of the second back-up ring is a surface with a convex circular cross-section, the end surface portion of the second back-up ring is a surface with a concave circular cross-section, and the seal ring is an O-ring.