A bottle cap is disclosed, which displays an opened state and cannot contain contents again once it is opened. The bottle cap includes a cap having a screw formed on its inner surface to be engaged with a screw formed on an outer surface of a nozzle of a bottle, a reverse moving means connected to the cap and ascending in a direction opposite to a descending direction of the cap as the cap is rotated downwardly, a rod shaped nut having a screw engaged with the reverse moving means and moving up and down in a screw direction without rotation, and a display installed in the lower portion of the nut, displaying an opened state of the cap by hanging a part of the cap in the nozzle as the cap is moved downwardly by being pushed by the nut if the cap is opened.