A thermochromic writing instrument, comprising: a writing body, storing a thermochromic ink therein, stored in a barrel; an operation body provided in the barrel to be operated to longitudinally move the writing body; a nib of the writing body configured to be capable of appearing from a front end of the writing instrument; a friction unit, rubbing handwriting in the thermochromic ink and capable of thermally changing the color of the handwriting by frictional heat generated at that time; and only a single clip body, wherein the friction unit is provided at a rear end of the barrel, the single clip body is provided in a side wall of the barrel, the single clip body comprises the operation body, and the writing instrument further includes a longitudinally extending slide hole is provided in a side wall of the barrel, characterized in that an appearance mechanism putting the nib of the writing body from a retraction condition into a projection condition by slide operating the single clip body placed in the slide hole forward, wherein the writing instrument includes a cam mechanism in the barrel comprising the appearance mechanism, the cam mechanism includes a cam and a projection capable of alternately engaging with each other at longitudinally different positions, and by slide operating the clip body forward, the cam and the projection are alternately engaged with each other at longitudinally different positions to alternately put the nib of the writing body in the projection condition or the retraction condition and wherein the barrel includes therein, a plurality of longitudinally extending cam teeth and cam grooves alternately placed along a circumferential direction, a rotary member having a plurality of projected ribs rotatably placed in a rear of the writing body and capable of alternately engaging with the cam teeth or the cam grooves, another plurality of cam teeth provided in the clip body and turning the rotary member, and an biasing member biasing the writing body backward, which comprise the appearance mechanism, and by slide operating the clip body forward against a biasing force of the biasing member, the projected ribs are alternately engaged with the cam teeth or the cam grooves to alternately put the nib of the writing body in the projection condition or the retraction condition. a writing body, storing a thermochromic ink therein, stored in a barrel; an operation body provided in the barrel to be operated to longitudinally move the writing body; a nib of the writing body configured to be capable of appearing from a front end of the writing instrument; a friction unit, rubbing handwriting in the thermochromic ink and capable of thermally changing the color of the handwriting by frictional heat generated at that time; and only a single clip body, wherein the friction unit is provided at a rear end of the barrel, the single clip body is provided in a side wall of the barrel, the single clip body comprises the operation body, and the writing instrument further includes a longitudinally extending slide hole is provided in a side wall of the barrel, characterized in that an appearance mechanism putting the nib of the writing body from a retraction condition into a projection condition by slide operating the single clip body placed in the slide hole forward, wherein the writing instrument includes a cam mechanism in the barrel comprising the appearance mechanism, the cam mechanism includes a cam and a projection capable of alternately engaging with each other at longitudinally different positions, and by slide operating the clip body forward, the cam and the projection are alternately engaged with each other at longitudinally different positions to alternately put the nib of the writing body in the projection condition or the retraction condition and wherein the barrel includes therein, a plurality of longitudinally extending cam teeth and cam grooves alternately placed along a circumferential direction, a rotary member having a plurality of projected ribs rotatably placed in a rear of the writing body and capable of alternately engaging with the cam teeth or the cam grooves, another plurality of cam teeth provided in the clip body and turning the rotary member, and an biasing member biasing the writing body backward, which comprise the appearance mechanism, and by slide operating the clip body forward against a biasing force of the biasing member, the projected ribs are alternately engaged with the cam teeth or the cam grooves to alternately put the nib of the writing body in the projection condition or the retraction condition.