A writing instrument cap having a clip as a separate part. The clip attachment, to which the clip is fitted, is projectively formed on the outer periphery of the cap. The clip attachment is formed with a vent hole penetrating between the interior and exterior of the cap and raised portions for attachment of the clip. The raised portions are provided on both sides of the vent hole, with respect to the circumference of the cap. When the clip is attached to the clip attachment, the clip covers the clip attachment and the vent hole and creates a clearance between the clip interior and the clip attachment so as to establish an air passage that communicates between the vent hole and the outside atmosphere.