PURPOSE: To enable an observation of the state that a container is unsealed, by the visual observation from the outside.

CONSTITUTION: A ring-form score line 24 for forming an opening is made on the top wall 21 of a container. A pierced hole 52 is provided at the bottom wall of the fitting recess 51 of a socket 41. A downward swelled part 62 inserted in the pierced hole 52 is provided at the lower face of the fitting protrusion 61 of a plug 42. The outside part from the pierced hole 52 at the lower face of the bottom wall in the fitting recess 51 is fixed to the outside part from the score line 24 of the upper face of the container top wall 21. The lower face of the swelled part 62 is fixed to the inside part from the score line 24 at the upper face of the container top wall 21. A lower flange 53 is provided at the outer periphery of the socket 41. An upper flange 63 extended to the upper side of the lower flange 53 is provided at the plug 42. A skirt 65 is provided at the front end of the upper flange 63. An inwardly protruded line 67 fitted to the lower flange 53 from the underside is provided at the inside face of the skirt 65. A score line 68 for separation of the protrusion is provided above the protrusion 67 of the skirt 65.