A seal structure sealing a fitted part between the guide surface of a bearing part and a shaft part to prevent a fluid from flowing in a direction from a high-pressure fluid side to a low-pressure fluid side. The seal structure comprises a seal ring storage part having a radial contact surface surrounding the shaft part which is formed on the high-pressure fluid side of the bearing part and a seal ring disposed in the seal ring storage part, allowing its side surface facing the contact surface to come into contact with the contact surface, and allowing its inner peripheral surface to be fitted to the outer peripheral surface of the shaft part. The seal ring is formed in such a relative dimension that, even if elastically deformed by the pressure of the sealed fluid, the elastically deformed expanded portion thereof is not brought into contact with the peripheral surface of the seal ring storage part.