A sealing device suitable for being installed to an installation groove which is provided in an outer peripheral surface of one member among two members (1, 2) relatively moving, and sealing between said two members (1, 2) by coming into close contact with the other member which is positioned in an outer peripheral side of said installation groove and one side wall of said installation groove, wherein the sealing device comprises: a seal ring of rectangular shape having an outer peripheral surface of cylindrical surface shape which slidably comes into close contact with said other member, an end surface of axially vertical plane shape which is provided in one side in an axial direction and comes into close contact with the one side wall of said installation groove, an inner peripheral surface of cylindrical surface shape, and an end surface of axially vertical plane shape which is provided in the other side in the axial direction, and made of a resin; a support ring having a pressing surface which comes into contact with an inner peripheral corner portion provided in the other side in an axial direction in said seal ring and is formed into a taper surface shape, is combined with said seal ring and is constructed by a rigid member such as a metal; and a spring means which presses said support ring and the seal ring, and wherein a collapsing margin in relation to said taper surface shaped pressing surface in said support ring is normally set in an inner peripheral corner portion in the other side in said axial direction in said seal ring, characterised in that said inner peripheral corner portion in the other side in said axial direction in said seal ring is formed into a round shape having a circular arc shape in its cross section, thereby normally setting the collapsing margin in relation to said taper surface shaped pressing surface in said support ring. a seal ring of rectangular shape having an outer peripheral surface of cylindrical surface shape which slidably comes into close contact with said other member, an end surface of axially vertical plane shape which is provided in one side in an axial direction and comes into close contact with the one side wall of said installation groove, an inner peripheral surface of cylindrical surface shape, and an end surface of axially vertical plane shape which is provided in the other side in the axial direction, and made of a resin; a support ring having a pressing surface which comes into contact with an inner peripheral corner portion provided in the other side in an axial direction in said seal ring and is formed into a taper surface shape, is combined with said seal ring and is constructed by a rigid member such as a metal; and a spring means which presses said support ring and the seal ring, and wherein a collapsing margin in relation to said taper surface shaped pressing surface in said support ring is normally set in an inner peripheral corner portion in the other side in said axial direction in said seal ring, characterised in that said inner peripheral corner portion in the other side in said axial direction in said seal ring is formed into a round shape having a circular arc shape in its cross section, thereby normally setting the collapsing margin in relation to said taper surface shaped pressing surface in said support ring.