a rear outer cylinder with an open front face;

an inner sheath unrotatably accommodated in said rear outer cylinder; and

a front outer cylinder with open faces on both ends, wherein: a mouthpiece and a writing core element can be thrust out of an opening formed on the front face of said front outer cylinder, said writing instrument further comprising:

a protrusion for restraining rotating range arranged on a front end face of said inner sheath; and

a refill sliding cam and a mouthpiece sliding cam formed on an inner face of said inner sheath, said writing instrument further comprising:

a refill sliding element and a mouthpiece sliding element accommodated in said inner sheath slidably in an axial direction and unmovably in a radial direction; and

protruded portions for sliding formed respectively on outer faces of said refill sliding element and said mouthpiece sliding element; wherein: said protruded portions for sliding are respectively fitted to said refill sliding cam and said mouthpiece sliding cam; and said mouthpiece sliding element is slidably and unrotatably connected to said refill sliding element, said writing instrument further comprising:

a connecting element connected to said inner sheath rotatably and unmovably in the axial direction; and

a rotating position restraining member arranged on an outer face of said connecting element for restraining a rotation of said inner sheath; wherein: said mouthpiece sliding element is slidably fitted to said connecting element, wherein:

when said rear outer cylinder is rotated in one direction relative to said connecting element, said inner sheath is rotated in the same direction together with said rear outer cylinder so that said mouthpiece and said writing core element are propelled forward and thrust out of said opening of said front outer cylinder; and

when said rear outer cylinder is further rotated in the same direction, said writing core element is propelled forward and thrust out of an opening on the front face of said mouthpiece.