A writing instrument excellent in handwriting density and temporal stability. The writing instrument includes a ball-point pen tip attached to the front end portion of the ink storage cylinder; and a ball-point pen refill having an ink composition for a writing instrument loaded therein. The ink composition for the writing instrument is a thermochromic ink containing at least a thermochromic microcapsule pigment. The writing instrument includes a pressurization mechanism for applying a pressure to the back end of the ink composition for the writing instrument.