Abstract: A composite writing instrument, in which a mechanical pencil unit of the type from which lead is ejected and a ball point pen unit are mutually diametrically opposed and held together within a lower tubular casing. A cap is provided accommodating a mechanism for engaging these units to shift them alternately in the axial direction when the cap is rotated around the axis, whereby the units are alternately advanced through a mouth of the writing tip of the lower tubular casing to a writing position or retracted therethrough to a storage position within said lower tubular casing. The mechanical pencil unit is so constructed that the cap is shiftable within certain limits in the axial direction relative to the lower tubular casing when the mechanical pencil unit is in the writing position, whereby the lead of the pencil unit is fed, whereas the cap is nonshiftably locked relative to the lower tubular casing when the ball point pen unit is in the writing position.