A sealing structure applied to a fuel supply apparatus of an internal combustion engine, the fuel supply apparatus including a delivery pipe, and a fuel injection valve, the delivery pipe including a mounting portion into which the fuel injection valve is inserted, the delivery pipe being configured to supply fuel pressure-fed from a fuel tank to the fuel injection valve, the fuel injection valve including a connection portion and a fuel introducing portion that are inserted into the mounting portion, the fuel introducing portion being provided to protrude from an end surface of the connection portion toward an upstream side in a direction in which fuel flows from the fuel tank toward the fuel injection valve, an outer diameter of the fuel introducing portion being smaller than an outer diameter of the connection portion, the sealing structure being configured to seal a gap between an inner circumferential surface of the mounting portion and the fuel injection valve, the sealing structure comprising: an O-ring into which the fuel introducing portion is inserted; and a back-up ring (21; 321) into which the fuel introducing portion is inserted, the back-up ring being disposed between the O-ring and the end surface of the connection portion, the back-up ring (21; 321) having a cylindrical shape, wherein an outer diameter of the back-up ring (21; 321; 421) is smaller than an inner diameter of the mounting portion, the back-up ring (21; 321; 421) includes a pressure-receiving surface (23; 323; 423) that is a surface located closer to the O-ring, and a seating surface (22; 322; 422) that is a surface located closer to the end surface of the connection portion, an inner diameter of the back-up ring (21; 321) is gradually increased from the pressure-receiving surface (23; 323; 423) toward the seating surface (22; 322; 422), and is greater than the outer diameter of the fuel introducing portion at any position on a central axis of the fuel injection valve, characterised in that in a section of the back-up ring (21; 321; 421) along the central axis of the fuel injection valve, an angle (θ1; θ11; θ41) defined between the pressure-receiving surface (23; 323) of the back-up ring (21; 321; 421) and an inner circumferential surface (28; 328; 428) of the back-up ring (21; 321) is smaller than an angle (θ2; θ12; θ42) defined between the pressure-receiving surface (23; 323; 423) of the back-up ring (21; 321; 421) and an outer circumferential surface (95; 97) of the fuel introducing portion, and in a direction orthogonal to the central axis, a length from an outer circumferential surface (26; 326; 426) of the back-up ring (21; 321; 421) in the seating surface (22; 322; 422) to the inner circumferential surface (28; 328; 428) of the back-up ring (21; 321; 421) in the seating surface (22; 322; 422) is shorter than a length from the outer circumferential surface (95; 97) of the fuel introducing portion in the end surface to an inner circumferential surface of the mounting portion. an O-ring into which the fuel introducing portion is inserted; and a back-up ring (21; 321) into which the fuel introducing portion is inserted, the back-up ring being disposed between the O-ring and the end surface of the connection portion, the back-up ring (21; 321) having a cylindrical shape, wherein an outer diameter of the back-up ring (21; 321; 421) is smaller than an inner diameter of the mounting portion, the back-up ring (21; 321; 421) includes a pressure-receiving surface (23; 323; 423) that is a surface located closer to the O-ring, and a seating surface (22; 322; 422) that is a surface located closer to the end surface of the connection portion, an inner diameter of the back-up ring (21; 321) is gradually increased from the pressure-receiving surface (23; 323; 423) toward the seating surface (22; 322; 422), and is greater than the outer diameter of the fuel introducing portion at any position on a central axis of the fuel injection valve, characterised in that in a section of the back-up ring (21; 321; 421) along the central axis of the fuel injection valve, an angle (θ1; θ11; θ41) defined between the pressure-receiving surface (23; 323) of the back-up ring (21; 321; 421) and an inner circumferential surface (28; 328; 428) of the back-up ring (21; 321) is smaller than an angle (θ2; θ12; θ42) defined between the pressure-receiving surface (23; 323; 423) of the back-up ring (21; 321; 421) and an outer circumferential surface (95; 97) of the fuel introducing portion, and in a direction orthogonal to the central axis, a length from an outer circumferential surface (26; 326; 426) of the back-up ring (21; 321; 421) in the seating surface (22; 322; 422) to the inner circumferential surface (28; 328; 428) of the back-up ring (21; 321; 421) in the seating surface (22; 322; 422) is shorter than a length from the outer circumferential surface (95; 97) of the fuel introducing portion in the end surface to an inner circumferential surface of the mounting portion.