Cap structure for vessel

A cap structure for a vessel, which is coupled with the vessel to open/close the vessel, the cap structure comprising:

• an inner cap having a cylindrical shape, coupled with an upper end portion of the vessel to open/close the vessel, and provided at an inner-diameter surface thereof with a mounting part having a predetermined width;
• an outer cap having a cylindrical shape and fitted around an outer-diameter surface of the inner cap such that the outer cap is coupled with the inner cap;
• a circular part inserted into the mounting part and mounted in the mounting part such that at least one portion of the circular part is inwardly spaced apart from the inner-diameter surface of the inner cap by a predetermined distance, in which the circular part comprises protrusions formed in a smoothly curved shape on opposite positions of an outer-diameter surface of the circular part such that the protrusions make contact with the inner-diameter surface of the inner cap, and a detachable groove formed at an inner-diameter surface of the circular part at positions perpendicular to the protrusions;
• a deformation member to deform a shape of the circular part from a circular shape to an oval shape, or from the oval shape to the circular shape;
• and
• a component inserted into the inner cap and the circular part and having a detachable protrusion circumferentially protruding from an outer-diameter surface of the coupling part formed at a lower portion of the component and inserted into the detachable groove.