To improve high-pressure sealing performance of a seal mechanism using both a backup ring and an O-ring.

In this seal mechanism 10, the O-ring 40 and the backup ring 50 are housed in a ring housing groove 22 of a shaft 20. In a high-pressure environment, the O-ring 40 receives pressure from the high-pressure side, and abuts on an inclined surface 54 of a projecting part 53 to expand the projecting part 53 while being compressed by being pressed toward the backup ring 50, and while making a ring center part abut on an abutting part 51 of the backup ring 50. Thereby, the projecting part 53 expands to close a space 33 on the low-pressure side to receive the O-ring 40, and prevents the O-ring 40 from projecting to the space 33 on the low-pressure side. In this case, since the projecting part 53 is formed of a hard elastic material relative to that of the abutting part 51, the projecting part 53 further surely keeps the expanded shape.