Writing instrument and pen tip part

A writing instrument comprising:

• a pen tip which includes a writing portion and has a directional specification for writing of the writing portion;
• a cylindrical inner cylinder which fixes the pen tip;
• a cylindrical outer cylinder into which the inner cylinder can be inserted and which includes a finger guide portion on an outer peripheral surface;
• a fixture which fixes the outer cylinder to the inner cylinder;
• and
• a barrel which is fixed to the inner cylinder, wherein
• the inner cylinder includes a seat portion on one end side and a male screw portion on another end side, the seat portion abutting on one end portion of the outer cylinder in an axial direction,
• the fixture is cylindrical and includes a female screw portion in an inner circumferential surface, the fixture abutting on another end portion of the outer cylinder, the female screw portion being engaged with the male screw portion,
• the inner cylinder includes a plurality of projection portions on a part of an outer circumferential surface that is between the seat portion and the male screw portion and that faces the outer cylinder, the plurality of projection portions being arranged in a circumferential direction, and
• the outer cylinder includes a plurality of grooves in a part of an inner circumferential surface that faces the plurality of projection portions, the plurality of grooves being arranged in the circumferential direction and adapted so that the plurality of projection portions are arranged in the plurality of grooves.