To sufficiently keep the airtightness by forming a cap chamber between an inner lid and an outer lid of a cylindrical cap body, and turning the inner lid in an interlocking manner with the turning of the outer lid to reduce pressure in a container.

In a cap body 3, an outer lid 7 is fitted to an inner lid 5, and for example, a base 1 is made of glass or hard resin, and the inner lid 5 is made of elastomer or soft resin, and the outer lid 7 is made of hard resin. The inner lid 5 holds an extension part in which a peripheral edge part of a bottom surface 5a is extended downwardly between a mouth part of the base 1 and a peripheral side 8 of the outer lid 7, and screws it, and a cap is attached thereto. A cap chamber 11 is formed between the inner lid 5 and the outer lid 7. An inner lid screw part 13 at a center of the inner lid 5 is screwed to an outer lid screw part 15 at a center of the outer lid 7. When the outer lid 7 is turned, the inner lid screw part 13 bites into the outer lid screw part 15 to interlock the inner lid 5 therewith. The inner volume of the cap chamber 11 is reduced, and the inner volume of the base 1 is increased to form the vacuum. The bottom surface 5a is deformed to allow the inner lid 5 to be closely attached to the base 1 to keep the airtightness.