The present invention provides a cap which is mounted to a container. The cap is provided with: an inner frame member which is formed in a cylindrical shape having an axial first end and an axial second end and which has formed on the second end side a flange section protruding outward radially an outer frame member which has an axial first end and an axial second end, is formed in a cylindrical shape covered over the outer periphery of the inner frame member, has an outer frame engagement section formed at the first end, and has a cover section formed at the second end and covering the outer periphery of the flange section and an elastic member which is gripped between the outer frame engagement section of the outer frame member and the first end of the inner frame member and which closes the first end of the inner frame member. The cap is further provided with a weld section where a part of the inner wall surface of the outer frame member and the flange section of the inner frame member are affixed by welding. A gap is formed on the outside radially of the weld section at a position between the cover section of the outer frame member and the flange section of the inner frame member. The gap extends from the weld section and is open axially on the second end side of the outer frame member.
