INNER PLUG, CAP AND COMBINATION THEREOF

PROBLEM TO BE SOLVED: To improve sealing function for reliably preventing liquid from leaking by improving strength of a protruding string against sealing pressure and preventing a cap from being broken. SOLUTION: The inner plug 12 has a circular swollen part 22 to be pushed against an inner peripheral face 2A of an opening 2 on an outer peripheral face 16A of an inner plug body 16 and a protruding string 20 having a trapezoidal cross section in plane contact with an upper end face 2B of the opening 2 on a lower face 18A of a flange 18. The cap 14 comprises a body 24 having a thread 28 with a trapezoidal cross section in plane contact with a screw groove 3 of the opening 2 inside and a top plate 26 integrally formed successively from the body 24. The cap 14 has a circular inner plug press 30 formed extending from an inner face 26A on a side of the inner plug of the top plate 26 toward the inner plug 12 to be engaged with an inner peripheral face 16B of the inner plug body 16 for pressing the inner plug body 16 against the inner peripheral face 2A of the opening 2, and also has a curve on an inner face 25 on a part connected between the body 24 and the top plate 26.