The cap assembly comprises an outer cap (1) having open top and bottom ends and an inner cap (2) having at its top a closed end (5) and at its bottom an open end (3) and coaxially connected to the outer cap by means of radially extending ribs (6) to define openings between the outer (1) and inner caps (2). Flange elements (4) are disposed below the openings for preventing the writing tip (10) from entering the openings and guiding it toward the inner cap. The flange elements are circumferentially spaced apart from one another by means of slits (7) in alignment with the ribs. The flange elements may extend obliquely upwards from the inner surface of the outer cap or may extend obliquely downwards from the open end of the inner cap.
