A thermochromic writing instrument containing a thermochromic ink, comprising a barrel and a writing body accommodated in the barrel, wherein - the thermochromic ink is contained inside the writing body, - a pen tip capable of ejecting the thermochromic ink is provided at a front end of the writing body, and - at least one friction portion having a low abrasion property capable of thermally changing color of the handwriting of the thermochromic ink by the frictional heat generated when the handwriting is rubbed with the friction portion is provided on an outer surface of the barrel, characterized in that - the writing body is accommodated in the barrel so as to be capable of moving in a longitudinal direction, - the thermochromic writing instrument further comprises an operation portion provided on the outer surface of the barrel so as to project radially outward from a side wall of the barrel, - the pen tip of the writing body is adapted to project from / retract into a front-end hole of the barrel by operating the operation portion, - the thermochromic writing instrument further comprises a side slide type projection/retraction mechanism which shifts the pen tip from the pen-tip-retracted state to the pen-tip-projected state by a forward press operation of the operation portion, - a first friction portion is provided at a rear end of the barrel, whereby said first friction portion and the operation portion are disposed independently of each other on the outer surface of the barrel, and - a second friction portion is provided at the front end of the barrel so that a front end of the second friction portion is located ahead of the front end of the pen tip in the pen-tip-retracted state, and the front end of the pen tip is located ahead of the front end of the second friction portion in the pen-tip-projected state, wherein the outer periphery of the front end of the second friction portion has a convex curved outer surface or an inclined outer surface, wherein a pen-tip-supporting portion of the barrel having the front-end hole is made of a hard material, and the second friction portion made of a soft material is provided on an outer surface of the front end of the pen-tip-supporting portion of the barrel, whereby the second friction portion and the writing body are maintained in a non-contact state, wherein an angle (α) defined between a contact line which contacts with the outer surface of the second friction portion and the front end of the pen tip in the pen-tip-projected state and an axial line of the pen tip is set to 10 degrees or more and 60 degrees or less, and wherein the second friction portion is provided circularly on the outer surface of the pen-tip-supporting portion. - the thermochromic ink is contained inside the writing body, - a pen tip capable of ejecting the thermochromic ink is provided at a front end of the writing body, and - at least one friction portion having a low abrasion property capable of thermally changing color of the handwriting of the thermochromic ink by the frictional heat generated when the handwriting is rubbed with the friction portion is provided on an outer surface of the barrel, - the writing body is accommodated in the barrel so as to be capable of moving in a longitudinal direction, - the thermochromic writing instrument further comprises an operation portion provided on the outer surface of the barrel so as to project radially outward from a side wall of the barrel, - the pen tip of the writing body is adapted to project from / retract into a front-end hole of the barrel by operating the operation portion, - the thermochromic writing instrument further comprises a side slide type projection/retraction mechanism which shifts the pen tip from the pen-tip-retracted state to the pen-tip-projected state by a forward press operation of the operation portion, - a first friction portion is provided at a rear end of the barrel, whereby said first friction portion and the operation portion are disposed independently of each other on the outer surface of the barrel, and - a second friction portion is provided at the front end of the barrel so that a front end of the second friction portion is located ahead of the front end of the pen tip in the pen-tip-retracted state, and the front end of the pen tip is located ahead of the front end of the second friction portion in the pen-tip-projected state,