To prevent a rear-end knock member of a writing implement from rotating in relation to a holder tube.

This structure for preventing rotation of the knock member with a clip of the writing implement is constituted of the holder tube 1 having a substantially triangular cross section, a writing body 2 provided slidably inside this tube, a means for operating the writing body, and a knock cap 6 for working the operating means from outside the rear end of the writing implement. A means for preventing the knock member from rotating in relation to the holder tube is provided between the outer wall of the knock cap and a corner part of the inner wall corresponding to each apex of the triangular cross section of the holder tube.