Abstract: A cap for a writing device which prevents drying of ink within a given writing device comprises a cap having a ventilating hole at the leading end thereof, an inner cap formed within the cap and having a ventilating hole at the leading end thereof, a spring room and a valve room formed within the inner cap, a valve cap having longitudinal channels and a deep recess and installed to open and close the ventilating hole by the help of a spring, and an annular step formed on the opening side of the inner cap for preventing the departure of the valve cap, whereby the valve cap blocks the ventilating hole upon coupling of a writing pen body to the cap to prevent a pressure variation within the cap and to prevent the evaporation-drying of the ink.