The object of this invention is to provide a structure of seal ring, and seal structure using the seal ring for flange joints of composite tanks and pipes, that prevents seal surface variations due to differences in thermal contraction between the seal ring and the tank/the flange joint, etc., that improves the resilience of the seal, and that prevents gas leakage due to poor sealing, in order to achieve the stable sealing. The seal ring (1) fits into a ring groove (15) formed in a sealed unit such as a tank unit (10) or a flange joint (11) to provide a tight seal to contain fluids. The heart-shaped seal ring (1) has three curved surface areas (3,4,5) that smoothly flow together, comprised of a first curved seal surface (5) which contacts the formed surface (100) of the ring groove (15), a second curved seal surface (4) which contacts the seal surface (11a) of a second member (11) (e.g. manhole cover), and a projecting curved surface (3) on the opposite side. The foregoing seal ring (1) is provided with a hollow cavity (2) formed with the three curved surfaces (3,4,5) of uniform thickness, and an indent (8) that passes through from the hollow to the fluid introduction side