A sealing device exhibits a sealing function even when it is in a state in which squeeze of a seal ring is reduced and in a low-temperature condition. A sealing structure is provided with a cylinder head (300) for an engine which has an injector mounting hole (310) and is exposed to high pressure gas; an injector which is mounted in the injector mounting hole (310); and a seal ring (100) made of resin which seals an annular gap between the injector mounting hole (310) and the injector (200), wherein a step having a larger diameter on a combustion chamber side (E) where the high pressure gas is present is formed on an inner peripheral surface of the injector mounting hole (310), and the seal ring (100) is attached to a position where the seal ring is in close contact with a step surface (310c) of the step.