A reversibly thermochromic aqueous ink composition comprising water, a water-soluble organic solvent, a reversibly thermochromic microcapsule pigment, a comb-shaped polymeric dispersant having a carboxyl group in a side chain thereof, a nitrogen- and sulfur-containing organic compound, and a water-soluble resin, wherein a reversibly thermochromic composition comprising (A) an electron-donating color-developing organic compound, (B) an electron-accepting compound and (C) a reaction medium that determines the temperature at which the color-developing reaction between the components (A) and (B) occurs is encapsulated in the reversibly thermochromic microcapsule pigment; a writing instrument produced using the ink composition; and a writing instrument set comprising the writing instrument and a friction body.