Provided is a cap structure of a vessel, which is coupled with the vessel to close or open the vessel. The cap structure includes an inner cap having a cylindrical shape and coupled with an upper end portion of the vessel to open or close the vessel, 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 component inserted into the outer cap and the inner cap, and a detachable unit to fixedly couple the component with the inner cap or the outer cap or separate the component from the inner cap or the outer cap without separating the inner cap or the outer cap from the vessel. The cap opens or closes the vessel by fixedly attaching the component to the cap disassembled from the vessel or separating the component from the cap.