A writing instrument comprises a refill disposed slidably along the axial direction in an outer cylinder, a rotating cam set provided in the vicinity of the rear portion of the refill, a weighting balance placed in the vicinity of cam bar of the rotating cam set, a spring member for energizing rearwards the refill, and a knocking portion for operating the cam bar of the rotating cam set. According to the above construction of the writing instrument, when the writing instrument is shaken with force exceeding a certain level, gravity balance functions to project and retract the refill from the outer cylinder, while the refill may also be projected and retracted by knocking motions of the knocking portion.