A synthetic resin cap for a carbonated beverage-filled container, which is configured to be mounted on a lip section of a container filled with a carbonated beverage, comprising: a top plate section; and a cylindrical section extending downward from a circumferential edge of the top plate section, wherein an inner seal projection fitted into the lip section and an outer seal projection configured to abut an outer surface of the lip section are formed at an inner surface of the top plate section, an abutting convex section configured to abut an inner surface of the lip section and seal the container is formed at an outer surface of the inner seal projection at a position spaced apart from an opening end section of the lip section toward a container main body, the outer seal projection has an inner surface having an inner diameter reduced toward a distal end, and a minimum inner diameter section, which is a lower end of the inner surface of the outer seal projection, abuts the outer surface of the lip section at a position spaced apart from the opening end section of the lip section toward the container main body, wherein a thin wall section that is thinner than other portions is formed at a position close to a base end section of the inner seal projection, the minimum inner diameter section of the outer seal projection is disposed at a position higher than a maximum outer diameter section of the abutting convex section of the inner seal projection, a thickness of the outer seal projection is gradually reduced toward the distal end up to the minimum inner diameter section, characterised in that the thin wall section has a thickness of 1 to 2.2 mm, a height difference between the minimum inner diameter section and the maximum outer diameter section is 2.5 mm or less, and in that a distal end surface of the outer seal projection is curved so as to be convex such that a diameter of the distal end surface gradually increases from the lower end of the inner surface downward toward an outer surface of the outer seal projection. a top plate section; and a cylindrical section extending downward from a circumferential edge of the top plate section, wherein an inner seal projection fitted into the lip section and an outer seal projection configured to abut an outer surface of the lip section are formed at an inner surface of the top plate section, an abutting convex section configured to abut an inner surface of the lip section and seal the container is formed at an outer surface of the inner seal projection at a position spaced apart from an opening end section of the lip section toward a container main body, the outer seal projection has an inner surface having an inner diameter reduced toward a distal end, and a minimum inner diameter section, which is a lower end of the inner surface of the outer seal projection, abuts the outer surface of the lip section at a position spaced apart from the opening end section of the lip section toward the container main body, wherein a thin wall section that is thinner than other portions is formed at a position close to a base end section of the inner seal projection, the minimum inner diameter section of the outer seal projection is disposed at a position higher than a maximum outer diameter section of the abutting convex section of the inner seal projection, a thickness of the outer seal projection is gradually reduced toward the distal end up to the minimum inner diameter section, characterised in that the thin wall section has a thickness of 1 to 2.2 mm, a height difference between the minimum inner diameter section and the maximum outer diameter section is 2.5 mm or less, and in that a distal end surface of the outer seal projection is curved so as to be convex such that a diameter of the distal end surface gradually increases from the lower end of the inner surface downward toward an outer surface of the outer seal projection.