Provided are a writing instrument produced at a lower cost than conventional thermochromic writing instruments utilizing frictional heat, slim like a pencil and not bulky because it contains an ink directly in a barrel thereof, and excellent in portability; and a set of the writing instruments.The writing instrument 1 is obtained by directly filling a barrel thereof with an ink 2 which becomes colorless or colored, or undergoes a color change by heating and an ink follower 5 following the ink with its consumption; firmly fixing a writing tip portion 3 to the end of the barrel directly or via a relay member; and disposing a friction body 6 having an elasticity at the rear end portion of the barrel. The writing instrument set has a plurality of the writing instruments respectively different from each other in color tone.