An airtight container for positive pressure beverage, comprising a beverage container having a body portion and a flange that is continuous with an upper portion of the body portion and defines an orifice, and a cap that is attached to the beverage container so as to seal the orifice, wherein the cap has: a liner that has a disk portion for covering the orifice of the beverage container, an annular outer circumferential portion connected to an outer circumferential edge of the disk portion, the outer circumferential portion including a thick liner portion for sealing an upper side portion of the flange; and an exterior body that is fixed to the flange so as to cover the liner from an outer side, in order to seal the orifice together with the liner, wherein the exterior body presses the thick liner portion against the upper side portion of the flange and brings the thick liner portion in close contact therewith, wherein an inward sloping surface that slopes inwardly is formed on a lower surface of the outer circumferential portion at a radially inward region, the inward sloping surface configured to be in close contact with an inner corner portion of an upper surface of the flange, wherein an outward sloping surface that slopes outwardly is formed on a lower surface of the outer circumferential portion at a radially outward region, the outward sloping surface configured to be in close contact with an outer corner portion of the upper surface of the flange, wherein the thick liner portion extends downward from the outer sloping surface of the outer circumferential portion, wherein the liner is made of an elastic material, and the exterior body is made of metal or a plastically deformable material having a higher gas barrier performance than the liner, wherein the exterior body extends further downward than the liner, to an extent that an entirety of the liner is confined within a space formed between the exterior body and the beverage container, characterized in that the cap further comprises: a thin liner portion for sealing a lower side portion of the flange, wherein the thin liner portion extends downward from the thick liner portion, wherein the exterior body presses the thin liner portion against the lower side portion of the flange and brings the thin liner portion in close contact therewith, and wherein the thickness of the thin liner portion is 0.2 to 0.25mm, and a ratio of the thickness of the thin liner portion to a thickness of the thick liner portion is 0.4 to 0.5. a liner that has a disk portion for covering the orifice of the beverage container, an annular outer circumferential portion connected to an outer circumferential edge of the disk portion, the outer circumferential portion including a thick liner portion for sealing an upper side portion of the flange; and an exterior body that is fixed to the flange so as to cover the liner from an outer side, in order to seal the orifice together with the liner, wherein the exterior body presses the thick liner portion against the upper side portion of the flange and brings the thick liner portion in close contact therewith, wherein an inward sloping surface that slopes inwardly is formed on a lower surface of the outer circumferential portion at a radially inward region, the inward sloping surface configured to be in close contact with an inner corner portion of an upper surface of the flange, wherein an outward sloping surface that slopes outwardly is formed on a lower surface of the outer circumferential portion at a radially outward region, the outward sloping surface configured to be in close contact with an outer corner portion of the upper surface of the flange, wherein the thick liner portion extends downward from the outer sloping surface of the outer circumferential portion, wherein the liner is made of an elastic material, and the exterior body is made of metal or a plastically deformable material having a higher gas barrier performance than the liner, wherein the exterior body extends further downward than the liner, to an extent that an entirety of the liner is confined within a space formed between the exterior body and the beverage container, characterized in that the cap further comprises: a thin liner portion for sealing a lower side portion of the flange, wherein the thin liner portion extends downward from the thick liner portion, wherein the exterior body presses the thin liner portion against the lower side portion of the flange and brings the thin liner portion in close contact therewith, and a thin liner portion for sealing a lower side portion of the flange, wherein the thin liner portion extends downward from the thick liner portion, wherein the exterior body presses the thin liner portion against the lower side portion of the flange and brings the thin liner portion in close contact therewith, and wherein the thickness of the thin liner portion is 0.2 to 0.25mm, and a ratio of the thickness of the thin liner portion to a thickness of the thick liner portion is 0.4 to 0.5.