The tubular main body of a writing instrument has a cylinder the writing end of which is formed to include a cap-fitting portion. A gripping portion, which is formed on the cylinder below the cap-fitting portion, has an outer peripheral surface provided with a number of annular slip-preventing grooves. A cap detachably fitted on the cap-fitting portion of the cylinder includes a clip extending axially to a point in the vicinity of the gripping portion. The clip has an engaging projection formed on an underside thereof for engaging one of the slip-preventing grooves of the gripping portion when the cap is attached. When the writing instrument is clipped to the fabric on the outer side of a shirt or jacket pocket, the engaging projection on the clip and the slip-preventing groove of the gripping portion on the cylinder of the tubular main body engage each other via the fabric clamped between them.