Retractable capless marking pen

A knock-type writing instrument comprising:

• (a) a sheath having a back end and a front end;
• (b) a writing member slidably accommodated in said sheath and having a front portion of relatively small diameter and a rear portion of relatively large diameter;
• (c) actuating means on said sheath back end for actuating said writing member between a writing position and a non-writing position;
• (d) protrusion means mounted within said sheath in the front end thereof;
• (e) an elastic cylinder having an annular opening and covering a part of said relatively small diameter portion of said writing member;
• (f) an elastic pressing member surrounding said elastic cylinder;
• and
• (g) a slanting surface on the periphery of said elastic pressing member and engageable with said protrusion means, engagement of said slanting surface with said protrusion means causing said elastic pressing member to contract about said elastic cylinder to thereby close said annular opening and seal said elastic cylinder about said writing member, and disengagement of said slanting surface from said protrusion means allowing said elastic pressing member to expand, thereby opening said annular opening and elastic cylinder and allowing said writing member to slide to said writing position.