A thermochromic writing instrument and a writing body fitted to said thermochromic writing instrument, wherein the writing body comprises an ink storage tube having an outer diameter smaller than an inner diameter of the rotary member; a nib press fitted and fastened into a front end opening of the ink storage tube; a thermochromic ink filled in the ink storage tube; and a tail plug attached at a rear end opening of the ink storage tube, wherein the tail plug has about the same maximum outer diameter as an outer diameter of a rear of the ink storage tube, and is provided with a vent communicated with the inside of the ink storage tube. The writing body is stored in a barrel so as to become longitudinally movably in a backwardly biased condition; both the rear end of the ink storage tube and the tail plug are inserted into the rotary member; and the tail plug abuts on an inner surface of the rotary member.