A sealing device fitted in a fitting groove provided in an opposing surface of an inner circumferential side member among the inner circumferential side member and an outer circumferential side member that move relative to each other and face each other, and slidably closely contacting an opposing surface of the outer circumferential side member, thereby suppressing leaking of a sealing target from a high pressure side to a low pressure side between opposing surfaces of the two members; the sealing device comprising: the inner circumferential side member; the outer circumferential side member; a combination of an inner seal ring disposed on an inner circumferential side of the opposing surface of the outer circumferential side member, an outer seal ring held in a holding groove provided in an outer circumferential surface of the inner seal ring and slidably contacting the opposing surface of the outer circumferential side member, and a back ring disposed on an inner circumferential side of the inner seal ring; the inner seal ring being formed from a material having higher rigidity than the outer seal ring; the back ring being formed from a material having higher elasticity than the outer seal ring and the inner seal ring and having elasticity to press the outer seal ring against the opposing surface of the outer circumferential side member via the inner seal ring; and characterised by: the inner seal ring including a cut portion at one location on a circumference of the inner seal ring; the outer seal ring being formed from a material having lower friction and lower rigidity than the inner seal ring and being formed endless circumferentially; and the outer seal ring being configured to satisfy a relationship: T > A where T is a radial thickness dimension of the outer seal ring, G is a protrusion gap dimension in the sealing device defined by the radial direction dimension of the gap between the inner and the outer circumferential members (41, 43), R is a radial width dimension of an open side edge chamfered portion of a seal fitting groove provided in the outer circumferential surface of the inner circumferential side member, and A is a sum of the protrusion gap dimension G and the radial width dimension R of the open side edge chamfered portion. the inner circumferential side member; the outer circumferential side member; a combination of an inner seal ring disposed on an inner circumferential side of the opposing surface of the outer circumferential side member, an outer seal ring held in a holding groove provided in an outer circumferential surface of the inner seal ring and slidably contacting the opposing surface of the outer circumferential side member, and a back ring disposed on an inner circumferential side of the inner seal ring; the inner seal ring being formed from a material having higher rigidity than the outer seal ring; the back ring being formed from a material having higher elasticity than the outer seal ring and the inner seal ring and having elasticity to press the outer seal ring against the opposing surface of the outer circumferential side member via the inner seal ring; and characterised by: the inner seal ring including a cut portion at one location on a circumference of the inner seal ring; the outer seal ring being formed from a material having lower friction and lower rigidity than the inner seal ring and being formed endless circumferentially; and the outer seal ring being configured to satisfy a relationship: T > A where T is a radial thickness dimension of the outer seal ring, G is a protrusion gap dimension in the sealing device defined by the radial direction dimension of the gap between the inner and the outer circumferential members (41, 43), R is a radial width dimension of an open side edge chamfered portion of a seal fitting groove provided in the outer circumferential surface of the inner circumferential side member, and A is a sum of the protrusion gap dimension G and the radial width dimension R of the open side edge chamfered portion. the inner seal ring including a cut portion at one location on a circumference of the inner seal ring; the outer seal ring being formed from a material having lower friction and lower rigidity than the inner seal ring and being formed endless circumferentially; and the outer seal ring being configured to satisfy a relationship: T > A where T is a radial thickness dimension of the outer seal ring, G is a protrusion gap dimension in the sealing device defined by the radial direction dimension of the gap between the inner and the outer circumferential members (41, 43), R is a radial width dimension of an open side edge chamfered portion of a seal fitting groove provided in the outer circumferential surface of the inner circumferential side member, and A is a sum of the protrusion gap dimension G and the radial width dimension R of the open side edge chamfered portion.