To prevent a seal-ring 3 from being damaged or causing dust by a plasma within a sealed space S or corrosive fluid, or from being broken by excessive compression.

A sealing structure comprises a dovetail seal attaching groove 12, the seal-ring 3 in which side surface expansion sections 3a, 3b are held within this seal attaching groove 12, and a head section 3d consists of rubber-like elastic material projected from the seal attaching groove 12, and a spacer 4 which is held in the seal attaching groove 12 and located at sealed space S side, and consists of a material greater in durability than the seal-ring 3. Furthermore, the spacer 4 comprises a collar section 41 supported on a groove shoulder 124 of the sealed space S side of the seal attaching groove 12, and a groove inner-side inserting section 42 fitted in an inner-side surface 122 of the seal attaching groove 12 and pushing the side surface expansion section 3b of the seal-ring 3 to an inner-side surface 123 of the groove shoulder 125 on the side opposite to the sealed space and a shoulder section 43 between the collar section 41 and the groove inner-side inserting section 42. Additionally, the thickness t of the collar section 41 is smaller than a projection height h1 from the seal attaching groove 12 of the seal-ring 3 when non-compressing.