Knocking writing utensil

A knocking writing utensil comprising:

• an external cylinder having a guide hole formed therein;
• a writing medium housed in the external cylinder so as to be movable in an axial direction;
• an elastic body for urging the writing medium rearwardly;
• a knock body exposed outwardly from the external cylinder to be knocked, wherein the knock body operates to move the writing medium between a withdrawal position at which the writing medium is withdrawn into said external cylinder and a forward position at which a tip of said writing medium protrudes from a tip of said external cylinder;
• a clip with its proximal end fixed to said external cylinder;
• an engagement body, provided within said external cylinder, for positioning the writing medium;
• an engagingly-locking protrusion formed on said engagement body;
• an engaging-lock receiving protruding portion including an engaging-lock receiving portion at its front end being formed on a back surface of a non-proximal end of said clip, said guide hole being formed in the external cylinder so as to correspond to said engaging-lock receiving protruding portion of the clip, wherein the guide hole is wider than said engaging-lock receiving protruding portion in only one lateral side of the guide hole relative to said engaging-lock receiving protruding portion, wherein said engagingly-locking protrusion is engaged with said engaging-lock receiving portion through said guide hole to hold the writing medium at the forward position and is disengaged from said engaging-lock receiving portion and placed behind said engaging-lock receiving protruding portion to hold the writing medium at the withdrawal position;
• and
• a knock bar, disposed inside said external cylinder, being integratedly operated with the knock body and including a cam portion,
• said cam portion being engageable with said engagement body,
• said cam portion guiding said engagingly-locking protrusion beyond said engaging-lock receiving protruding portion from behind it to said engaging-lock receiving portion and guiding said engagingly-locking protrusion from said engaging-lock receiving portion to behind said engaging-lock receiving protruding portion so as to pass by one lateral side of said engaging-lock receiving protruding portion.