A sealing structure includes an O-ring and a back-up ring. An outer diameter of the back-up ring is smaller than an inner diameter of a mounting portion of a delivery pipe. An inner diameter of the back-up ring is gradually increased from a pressure-receiving surface toward a seating surface, and is greater than an outer diameter of a fuel introducing portion of a fuel injection valve. In a section of the back-up ring, an edge angle defined between the pressure-receiving surface and an inner circumferential surface is smaller than a gap angle defined between the pressure-receiving surface and a tapered outer surface of a tapered portion in the fuel introducing portion. A dimension of the seating surface in the back-up ring is shorter than a dimension in an end surface.